Kevin grinned as he noticed John starting to slowly grow smaller. It wasn't much - not yet - but the thrill of changing him where John didn't even notice shot through him.
The jock laces his hands through John's thick hair, pulling him off his cock, smile fixed firmly in place.
"That was impressive..." he purrs, "Let's see what that dick of yours can do then." Kevin pulls John closer, grinding against him as he takes his own fingers into his hole. He moans, the former delivery man grinding back, pleasure traveling up and down his spine at the contact, heat pooling in his belly.
Kevin doesn't bother with lube, stretching himself out rather quickly. It might be a wise idea to take his time, but he was much to eager to get started on the next phase of his plan - the one after getting the other man's cock in him.
He grabs the smaller man's cock, a glint in his eye, and slowly guides it towards his hole. As the tip kisses him, he feels John's shaft twitch in anticipation. Kevin takes a deep breath, slowly letting John enter him.
John, while ecstatic and harder he could ever remember being, was still shocked that Kevin was willing to allow John to fuck him. Not only did Kevin seem so - so dominate, so unwilling to let John view him as an equal, but something wasn't reading right to him.
But, as Kevin takes him to the hilt, his full ass touching John's balls, John can't keep his hips from twitching, his mind from going blank. Kevin groans languidly, limply laying on the bed as he allows himself a beat to adjust to the cock buried deep inside him.
"Yeeesssssss," he moans, "you'll do absolutely perfectly." The jock slowly loops his legs around John's waist, tightening him to keep them together, and starts to move. John shudders at how good he feels, the bigger man wrapped around him, but a part of him know he's not in control, and it makes him twitch and moan.
As Kevin slowly fucks him, John slowly beginning to help set the pace, the changes start.
John's cock lengthens, and he groans in pleasure as he thickens, Kevin's hole almost becoming painfully tight. He shudders as his balls enlarge, as he feels a more and more desperate need to cum. He starts to shrink even more, needing to clamber onto the bed to keep himself in the jock under him.
He keeps getting longer, thicker, as the rest of his gets smaller and smaller, picking up the rather slow pace that he had set before. Soon, he's ploughing Kevin into the mattress, the force behind his thrusts slowly fading away. John gasps, feeling his orgasm begin to crash over him, and Kevin sits up, pinning the other man under him.
John's small now, his long cock embedded in Kevin being larger than him, and as he finally empties his now much larger balls, he starts to shrivel away even more.
Before long, John finally starts to process again through his post-orgasmic haze. He's blinded and yet being gripped so, so tight, something vice-like surrounding his entire body. Slowly, he feels what's surrounding him move, something gripping where his feet should be, pulling him out of this heaven, before slamming him back in. He hears a distant, near thunderous, moan, before he's fucking some sort of hole again.
He twitches, trying to reach around, only to find that his hands are missing, but successfully curves, hitting soft and pliant walls at a new angle, to the great and obvious pleasure of whatever he's being fucked into. He's pulled completely out of the hole, light surrounding him as he tries to see what's happening, only to see a tight ass in front of him for less than a second before he's thrust in again.
This treatment continues for a long while, the entirety of him being hammered into someone much, much larger, before the walls around him convulse as whoever he's in finally cums.
John is gingerly pulled out and tossed aside, landing on the soft mattress. As he tries to get his bearings, he sees Kevin. The jock, who was large before, is absolutely massive now. And distantly, on the wall, is a mirror.
And the only thing on the bed is a flesh-colored dildo, at least a foot long.