You're sitting around in tails lab as tails was making some sort of potion with blue liquid before the bottle cracked as the liquid splashed on him, absorbing into his body

"..o-oh no-"

Tails said surprised before his body begane to grow, his arms becoming large and muscular, his chest puffing out large pecs, getting a 8 pack, muscular legs and even his tails became giant and fluffy

"..woaahhh!"

Tails said while looking at his new extremely buff body