Taken Back

Sarah, in Jon's body, grabbed the rock from the floor.
"Give it here, Sarah," said Jon, "and I'll try to find someway to help you."
"I can help myself!" shouted Sarah in tears.
"Sarah! I can't let you have that rock!" shouted Jon back at her, moving to grab the rock. With his huge build, Jon could have easily forced the rock from the hands of his scrawny former body.
"I wish you would freeze right there!" said Sarah as the rock glowed.
Jon was unable to move, but he could still talk. "Don't do anything stupid, Sarah! Look at the mess we're already in!" he said.
To her credit, Sarah had a plan. "I wish Jon's old body looked exactly like my old body," she said.
The rock glowed and Sarah was engulfed with green light as her body morphed in the voluptuous shape of her old self. Still in John's clothes, Sarah smiled with delight.
"You idiot!" shouted Jon, furious, "When I take the test we'll change back and I'll be stuck as you and you'll still be stuck with this body!"
Sarah didn't seem to listen, she just looked down, admiring her body. "I wish I wasn't wearing these clothes, though," she said.
There was a flash and her clothes vanished completely. Suddenly, all thoughts in Jon's head went to staring at Sarah's perfect form. Though he still couldn't move, he noticed pressure build in his pants. So did Sarah. With a wicked smile she bent over and pulled them down, exposing his massive erection.
"That's much better," she grinned. Jon realized it was no accident she wished her clothes away, it was just her teasing him. After all, she was probably very attracted to his new form.
Sarah knelt by his crotched and examined his erection.
"I think maybe it's too big," she grinned, as she rubbed it with her hand. "I wish Jon would get smaller every time I say his name. How was that wish, Jon?"
The rock glowed and John felt himself shrink. Not much, just over an inch in height. He was still nearly a foot taller than Sarah and his penis stretched 12 inches long easily.
"Jon, Jon, Jon. I wish you would fuck me, Jon," she smiled as he shot down in size.
Suddenly able to move again, John grabbed Sarah, pushed her to the bed, and pushed into her. She moaned with pleasure and giggled with delight as she called out his name with every thrust. Smaller and smaller, Jon continued to push into Sarah. He was so focused on the sex, he didn't notice as he dropped past her height and continued to shrink. Finally, he came and realized the world had gotten much, much larger!
He was now only inches in height, pushing himself into a vagina nearly the size of his own body. With long, slender fingers, Sarah reached down and picked the tiny man from her crotch. She lay, naked on Jon's bed and placed the tiny bodybuilder between her breasts.
"This size suits you," she smiled, "I guess I'll have to call you something else now. How about 'Slave'?"
Jon was furious, the haze of sex leaving his mind. He laughed at her, "Sarah, you idiot! We're going to switch bodies back! You'll be stuck at this size!"
"Not if you can't take the test," she grinned as a look of horror spread across Jon's face...