Las Lindas Male Expansion, an interactive fiction on Writing.com. An excerpt:
Minos looks down, his eyes widening as he sees his abs disappearing, being replaced by a flabby belly. "Wh-What is this?", he wonders out loud. Mora grins and giggles, rubbing Minos' gut. "A special formula that makes the user grow massively fat!", she proudly announces. "I've decided I want you to be incredibly fat, my dear bull." "But why?", Minos questions her, feeling his chest develop into a nice soft pair of moobs. "I can't quite explain it—something to do with whatever Alej slipped me.", Mora replies. The two bovines watch as Minos' body grows fatter and fatter, his clothing growing tighter. Finally, his belly flops out of his shirt, allowing Mora to rubs it more directly, and his pants begin to pop at the seams in the back. "Mmmm, yeah, NOW you're making progress…", Mora murrs, noticing Minos' limbs becoming significantly less muscular.
This was an accidental find but ha ha, wow. (Also, I'm still unfamiliar with wiki tags, how do I put multiple paragraphs with line breaks into one quote?