Nikki made up her mind for what she was going to do next and decidedly slapped her ass. She smiled as her big round butt wobbled around. Knowing that her tiny friend was taking the brunt of her butt amused her. She knew he was down there, probably coughing on her recent gassy release, but what she didn't know was she shrunk him too much, to a nearly microscopic size. She picked the portal device back up, taking it off the bed and accidentally pressing a few more buttons on the side.

"TIME SLOW DOWN: OFF"

Mark felt his stomach lurch signifying his return to a normal speed of time. Things weren't going in slow motion anymore, not that he could really tell a difference from his surroundings. He was thoroughly lodged within the folds of his mischievous friend's anus, shrunken down to a size where he was powerless to fight back against her grasp. Just then he heard Nikki's voice, muffled from inside her butt, but still booming loudly up above.

"This portal device is amazing Mark! I'm surprised it works so well. You're so small I can barely feel you in my ass crack! Or maybe that's just because my butt's so big, hee hee" she laughed.

"NO NIKKI! YOU MADE ME TOO SMALL! GET ME OUT OF HERE NOW!" Mark screamed, his faint voice not even traveling outside her ass.

"I'm going to do you a little favour, okay Mark? I know initially I said I'd only allow you to touch my ass for three minutes, but I'm soooo impressed that I think I'll extend your time for the rest of the day. We both know it's what you really want, it's what you chose remember?"

Mark's heart sank. She knew he was trapped in her ass but she was going to keep him there all day? Even if he wasn't micro-sized this would be too cruel.

"I've still got a few more classes left in the day, so I'll be heading to school alright? You have fun down there, but don't enjoy yourself too much." she said playfully as she got dressed again.

This time instead of wearing her sexy yoga pants she opted for just her panties and a skirt. Nothing would be better to tease Mark with than seeing his freedom, just a small layer of fabric away. He'd be stuck in her panties until she decided to let him out, left to face her humongous butt cheeks and unbearable weight as she sat on him (and maybe a few farts too). The times where she used to sit on his face to tease him were funny because she could feel him squirm under her ass. Today was bound to be even more enjoyable because her ass was so much bigger and he was so much smaller than her.

She could hardly contain her laughter as she walked to school. Her big fat butt wobbled around in her panties as she walked, surely crushing the tiny boy. Nikki's ass was bigger than most girls and the sheer size of it made her cheeks jiggle with every step. The way her huge pale cheeks slid against each other admittedly felt good and she hoped Mark was rolling around in deep the crack between them.

Mark wasn't dealing with the tumbling glory of Nikki's ass cheeks, but honestly he'd rather be there than where he currently was. The tight anus was cramped and smelled awful, with blazing heat to match an active sauna. Every breath Mark took smelled like leftover farts, it almost made him sick. It smelled so bad in her ass, if the anus swallowed him fully he didn't know what other horrors would lie within. He had to get out of here before that happened.

Only he was stuck. The stained hole had a firm grip on his body and worse yet, it seemed to grind him left to right his giant friend walked to school. He was wet, both from traces of Nikki's ass sweat and his own. Maybe he could use that to his advantage and slip out? No good, it wasn't enough lubrication to actually let him slide around.

One of his arms however, felt a different kind of wetness, stuck between two wrinkles of Nikki's asshole. Scooping it up in his hand he discovered it wasn't shit, but rather mucus-like slime, the kind that would normally coat the bowels to ease feces through the lower intestine. It seemed like Nikki had an overabundance of anal juices in her ass, and for the first time it looked like Mark had a stroke of luck.

He slathered the goopy slime all over his body. It was warm, and smelled absolutely horrible, but he had to deal with it if he wanted to escape. He coated himself until he could roll around in the sticky gunk, he was finally slippery enough to take action. Slowly he began to slide himself backwards, out of the massive anus. The lumpy pucker squeezed down on him, almost like it wanted him to stay inside, but the overbearing flesh only served to push him out more, aiding to his escape. Before long, Mark could feel fresh air on his legs, then his torso, and eventually his head.

The tiny boy didn't know what he was in for. Finally free of the teen girl's nasty asshole he found himself in a world bigger than he could have imagined. He hadn't experienced Nikki's plump ass cheeks in full yet. Sure he had seen their hills rise above him when she dropped him in her crack, but he hadn't experienced how it felt to be smushed between them. The continental weight and girth of her buttocks instantly overwhelmed his micro body, sucking him into a constantly-moving tornado of flesh.

The girl marched on, her huge white globular butt cheeks pulverized the tiny man between them. Mark screamed as he endured the wiggling booty, stuck in an unending ocean of salty skin. He couldn't tell where it stopped or where it ended, what way was up or what way was down. All he knew was that the trek to school was a lengthy one, and Nikki's butt would smother him the entire way. Her massive chubby buns pumped up and down in rhythm with her thick thighs. Mark belonged to her meaty crevice now, and for the many excruciating minutes to come...

Later Nikki arrived at school, in time for afternoon math class. She made her way to her seat and sat down as slowly as possible without garnering anyone's suspicion. Her butt slowly stuck out from within her short skirt, the massive cheeks spreading apart ever so gradually. She wanted Mark to fall to the bottom of her panties, so he would be crushed between the hard surface of her chair and her giant fat rump.

Mark did indeed fall to the bottom of her underwear, landing on the soft stitched fabric. Looking up he saw a daunting sight, Nikki's full spread ass, bent over and hovering above him. The monolithic globes took up the whole sky, dimly lit by the small beams of light that passed through her panties. It was so huge, so astronomically big that Mark couldn't see it all at once. Her ass extended infinitely far into the horizon, it was the size of a small town and gave Mark perspective as to how small and insignificant he truly was.

Nikki's hands quickly grabbed onto her buns and spread them apart as she sat. Mark's jaw dropped in reaction. She wanted to sit on him with her ass spread, so he'd be flattened under her filthy ass crack and anus.

"Holy shit..." were the only words Mark could meekly utter as the sky fell down and sat on him.

With a heavy creak, Nikki's fat ass was seated and ready for class. She was in a good mood today, it wasn't often she got to torture Mark like this, and only using her butt. As expected, Mark was pinned against the girl's puffy backdoor again, stuck between it and the hard unforgiving seat. This was going to be a long day.