Don't Change, Ogata-Kun! summary:

Which one is the real you?! "Shota <=> Muscle Man" transformation harem comedy!Highschooler Ryoutarou looks like a primary school boy, and all he wanted from his new school was to have a quiet and peaceful life. But on his first day in his new school the delinquent Mizuki calls him out and immediately destroys his dreams. Ryoutarou thinks he's in for a violent shakedown, but instead he's taken to a club room where girls with a certain sexual fetish gather....