# When Size Matters

By: TheDragonBoy

## Content

This can’t be happening, this can’t be happening! Mabel thought. She sat with her back up against a tree, somewhere out in the twilight of Gravity Falls’ forests, her head and legs pulled inside her colorful sweater as she did her best to hide away from the world. That day, everything in her life it seemed had turned for the worst. She had spent the whole day planning out her 13th birthday- the day she and her brother would finally become teenagers- only to find out that her two closest friends wouldn’t be able to come and that growing up apparently wasn’t the happy adventure she had been led to believe. Not only that, but that same day would also be the twins’ last day in Gravity Falls before they had to go back home, and leave all their new friends and recently-discovered family members behind.

But the part that upset her the most, was that now she would be going home alone.

Their great-uncle Ford had offered to make Dipper his apprentice. And Dipper had said yes! How could he leave her like that? How could he make her go home and face high school all alone? It was bad enough that she would be leaving all her new friends behind, but her own brother? He was the one person she could count on! And why did he get to stay in Gravity Falls anyway? It wasn’t fair! She had helped on their adventures just as much as he had! But Dipper was going to get everything he wanted and she was just going to be left behind?!

If only there was a way I could make him stay with me, at least then we’d be doing it all together… Mabel thought despondently. She let her mind drift back over all their shared exploits from the past summer: fighting giant monsters, giant robots and even a trans-dimensional triangle. Then of course there were their more private “adventures”. Despite the circumstances, she felt a slight tingle deep between her legs as she briefly recalled the many times her brother had “helped” her with her developing urges.

That’s when she got the idea. She clenched her lower muscles, feeling the sensitive tunnel at her crotch as it led up into her body, disappearing at its end somewhere deep inside. Yeah, that could work! She thought. Her face lit up with hope inside the confines of her handknit sweater. The answer was so obvious, her alpha-twin body gave her everything she needed to make sure Dipper stayed with her for as long as she wanted. All she needed was a little Gravity Falls help.

“Mabel!?” came a distant voice. She poked her head out of her sweater and looked off into the darkening forest. It was Dipper, probably trying to find her and calm her down. She put on a determined face, wiping a few tears from her eyes and stood up. She started back in the direction of the voice, her brother’s calls getting louder as she got closer.

Eventually she made out the lights coming from the Mystery Shack, and then Dipper, rushing around the outskirts of the woods and calling her name. He saw her as she approached and ran toward her. “Mabel, there you are!” Mabel glared at him. “Listen, I know you’re upset but-”. Dipper was interrupted by a sudden charge from his sister. She rushed at him angrily, shoving him aside when she reached him and heading straight for the door to the Shack. “Mabel, I’m sorry! Please, we can talk about this!” he called after her as she disappeared up the stairs.

Mabel threw open the door to their room and dove under her bed, reaching for her hiding place where she kept her second-favorite possession she had acquired over the summer (second only to her grappling hook, of course). She wrapped her hands around the familiar item and shimmied out from under the bed, standing back up just as her brother came running into the room behind her. “Mabel?” he asked, concerned.

She took a breath and tried to compose herself a bit before turning around to face him, the size-changing flashlight clutched conspicuously in her hands. “Mabel, what are you doing with that?” he asked, taking a step back. He could practically feel the turmoil radiating from her face, he felt like an idiot for handling the situation so badly, but now he was even more worried about how Mabel was planning to handle things.

“I am not going home alone!” Mabel yelled, pointing the flashlight at her brother and flicking the switch. Dipper barely had time to move before the magical light engulfed him and his entire world began to change, “growing” around him as he shrunk. He regained his bearings as quickly as he could and tried to make a dash for the nearest large object, trying to find some cover, but Mabel was on top of him in an instant, wrapping her hand around his tiny form.

Everything went dark for Dipper as his sister’s fingers closed around him. He was so small now that he could fit entirely in her fist. He felt a jerk as he was lifted off the ground and whisked up into the air, and a moment later he heard the door to the room slam shut, cutting off any easy escape or aid.

Mabel relaxed her grip and reached down with her other hand, barely managing to grab her brother’s tiny shirt between her thumb and forefinger. He flailed around angrily for a moment. “Mabel! You can’t just shrink me whenever we have an argument!” he yelled. “Oh, I’m gonna do more than shrink you,” Mabel replied, and Dipper watched as she reached down with her free hand and stuck her thumb past the inner band of her skirt, sliding it down her legs along with her underwear.

For Dipper, this was an unexpected twist. He craned his head to look up at his sister as he tried to figure out what she was planning. Mabel, on the other hand, knew exactly what she was doing, and immediately started lowering him toward her crotch. Dipper’s eyes snapped back down to her rapidly approaching nethers. “Mabel, wait, what are you doing?” he asked frantically as she bent her knees a bit, spreading her legs apart just slightly.

“Take a deep breath, bro,” she said, pausing for a moment as he dangled just in front of her exposed slit. “Mabel, no! Please, wait-” But Mabel wasn’t in a patient mood. In one motion she shoved her fingers between her waiting lips, pushing her brother as far in as she could.

She let him go inside of her tunnel, and slowly she started moving her fingers in and out, her toes curling as she began to feel the familiar pleasure growing between her hips. As her privates grew wetter she started picking up the pace, letting her eyelids droop and her legs bend even more. A faint rhythmic “schlick”ing sound began to fill the otherwise silent room as she started rubbing harder, followed soon after by the sound of her breathing as it grew heavier.

The pleasure she was feeling was nothing compared to the other times Dipper had been in there, but of course this time it wasn’t just about the pleasure. Normally when they did this, she shrunk him down just small enough to fit inside, leaving him big enough to press up against all sides of her passage and give her a nice, full feeling. But this time she had made him much smaller, small enough to fit places he could have never reached before…

As her vagina began to pulse in time with her thrusts, she could faintly feel her brother’s tiny form as it pressed up against her sensitive inner walls and occasionally brushed the tips of her fingers. Slowly but surely he was being drawn deeper and deeper, up towards the far end of her tunnel.

Inside, Dipper was being subject to the full brunt of his sister’s enthusiastic sex, but at this point that was something he was relatively familiar with. Having gone through this a few times before, he was somewhat used to the way her slick walls squeezed around him, pressing him deep into their forbidden depths. Whether Mabel had always been careful or just because the muscles weren’t quite strong enough, he had never come out of it with any injuries to speak of, except for some soreness.

But this time was different, and he knew it. Right from the beginning he could tell Mabel had made him much smaller than usual. He would never forget the first time he saw her lower lips up close- how big and intimidating they had seemed at his shrunken size- and the ones she had shoved him through a few moments ago- though they were undoubtably the same pair- had been much bigger. Not only that, but normally once he was inside, her contractions felt significantly harder than how they did now. And this time he could also feel his clothing and hair clinging firmly to her walls, just from the stickiness of her fluids, requiring a good amount of effort to peel off.

Of course that didn’t stop the eager tunnel from pulling him deeper, sliding his wet form further down its length with each contraction. And it hadn’t taken long for Dipper to figure out that that was exactly what Mabel wanted. His instincts told him he should struggle- to try and fight back and get out- but he knew better. After all he had been in her position before, he had felt his body as she was now, rubbing irresistibly along the hungry folds of her insides, and he knew perfectly well how easily she could keep him there. Even if he somehow managed to make it back to the outer limits, one good squeeze was all it would take to send him back inside. Plus, this time Mabel was using her fingers, and at the rate she was going now, getting too close to them might prove even more dangerous than her tightest contractions.

Indeed, Mabel was definitely enjoying herself, rubbing her fingers along her privates in ways only she was experienced enough to know. She slowly began to stumble back towards her bed, doing her best to maintain the friction between her legs. When she finally felt her exposed rear press up against the soft sheets, she let herself fall back onto the mattress, bringing her knees up to her chest and sticking her legs into the air as she feverously massaged her depths, her fluids now beginning to leak onto the bedding as she approached her climax.

Now that she was laying down, gravity was no longer working against her, and she felt Dipper slipping inward even easier. Half from instinct and half from intent she slammed her feet down onto the edge of her bed, lifting her butt off the sheets and arching her back. The new position meant that her vagina was now at a gentle incline, with her opening at the top and her most intimate reaches at the bottom.

She gave another deep thrust, barely missing Dipper’s feet, but stimulating her walls enough to bring on a sizable contraction, strong enough to send her brother down the tiny remaining distance left at the end of her tunnel.

Mabel forced herself to contain the scream that erupted from inside as she felt Dipper finally brush against the far end of her passage. The sensation brought with it another strong series of contractions, which only served to press her brother harder against the extra-sensitive flesh. With one great final push she rammed her fingers into her privates, burying a good portion of her hand between her lips as her orgasm squeezed tightly around it.

But nowhere did it squeeze as tightly as at her core, where she could feel Dipper pressed up against the entrance to her innermost chambers. She clenched her legs together tightly, hoping to add as much pressure to this final contraction as possible. For a long moment she felt her brother held tightly against the ring of her cervix, the pressure around him pushing him hard into the protective tissue, until finally she felt something shift, and just as her climax faded she felt Dipper disappear.

Panting, she blinked a few times and withdrew her hand from her crotch. She waited for a moment, “listening” to her insides for the sensation of her brother’s tiny form crawling its way out of her tunnel. But the feeling never came; he was really gone.

No, not gone, just moved.

Faintly, Mabel began to feel something somewhere new. It wasn’t nearly as defined as with her sensitive vagina, but she could just barely make out the feeling of something moving, somewhere deep inside her body, touching places that had never before been touched. She sat there for a long moment, until the motion finally stopped. But even when her occupant was still, she could still faintly perceive his presence, like a slight pressure in her belly.

After a while, she started to feel a little worried. After all Dipper wasn’t moving and hadn’t said a word since she had finished. “Dipper?” she asked hesitantly, still a bit breathless, “Are you okay in there?” There was a moment of nervous silence. “Yeah, I guess so.” Came a muffled, apathetic voice. Mabel breathed a sigh of relief, but the air quickly grew tense again as another silence followed.

“Look, Mabel-”

“Dipper, I-”

Both siblings stopped as they realized they were talking over each other. And there was yet another silence before Dipper decided to try talking again.

“Mabel, I’m not happy about what just happened, and I’m not happy about where I am right now. And I will definitely be taking that flashlight once this is all over. …But- like I’ve been trying to tell you this whole time- I’m sorry I told Grunkle Ford that I’d be his apprentice without talking to you first. You’re my sister and I should try to make decisions that are best for both of us…”

Mabel took a moment to take in what he was saying. She had just used him as a literal sex toy and he was still apologizing to her. Even now her brother still had her best interests at heart. She sniffled and held back a tear as she realized what an inconsiderate, selfish jerk she was being- to her own brother no less. “No, Dipper, I’m sorry, I shouldn’t have done this. I- I just really didn’t want to lose you…”

“I know. To be honest I hadn’t really stopped to think about how different my life would be without you. Everything was just so cool and exciting, but… it wouldn’t be the same if you weren’t around…” Dipper said, his words just faintly audible even in the otherwise silent room. At that point, one of them would have probably suggested one of their awkward sibling hugs, but the current arrangement was already way more “awkward” and way more “hug” than they could possibly ever top.

After another moment, Dipper finally spoke again. “So… this is your womb, huh?” “Yeah, I guess so…” Mabel replied, “What’s it like in there?” “Tight. But… nice,” he answered. Mabel sat up into a more comfortable position, producing a “woah!” from her occupant. “Sorry, I guess moving around is going to be a bit weird while you’re in there,” Mabel said.

“Mabel… you are going to let me out, right?” Dipper asked. “Yeah, totally… in a couple days,” Mabel answered. “What!? Seriously!?” “Well that was the plan. You’re stuck in there until my next- urm, you know. You should be able to get out then.” Mabel explained. “‘Until you next-’ Oh! Mabel! Gross!” Dipper exclaimed. “Yeah, sorry Dipper… But don’t worry, I’ll take good care of you until then!” Mabel said encouragingly. Dipper just groaned.

Bonus – The Next Morning

Mabel had woken Dipper a few minutes ago, just after waking up herself, to make sure he was alright after spending the night inside. Other than having to get used to his entire world turning every time his sister rolled over, he seemed to have done pretty well. Now as far as Dipper could tell, Mabel was getting ready for the day like she did normally.

He was still wiping the sleep from his eyes, in his dark, cramped world, when he heard the familiar “clank” of a plastic seat hitting a porcelain bowl. He didn’t think much of it at first, until he started to hear what sounded like a muffled sink draining from somewhere very nearby, followed by the unmistakable sound of a certain liquid pouring into said bowl.

“Ugh! Mabel!” he exclaimed, realizing what was happening. “What? I have to go!” she replied, continuing to relieve herself. Dipper heard a particularly loud squelch, followed by a distinct “phhhrt” that came from somewhere uncomfortably close by. “Mabel!” Dipper yelled again. He could feel the walls of his enclosure shifting slightly as the contents of his sister’s bladder and digestive tract made their way outside. “Well I can’t exactly hold everything in for the rest of the month,” Mabel insisted. Her statement was emphasized by a distant “plop” as something landed in the pool of water below.

“Oooh, I think I’m gonna be sick,” Dipper moaned. “You’d better not make a mess in there, Dipper! Besides, if you do you’ll have to live in it for the rest of your stay!” Putting aside her comic characterization of her womb as some sort of hotel room, Dipper decided to take her advice. He did his best to get a hold of himself as he heard the nearby store of urine drain to empty and felt a few more pieces of solid waste push past his chamber.

“Uhhhhg, this is going to be a long few days.”

Originally Published: 21-Aug-2018