Read Danberu Nan Kiro Moteru? manga
Sakura Hibiki is your average High-school girl, with a voracious appetite. Noticing her clothes tightening in lieu of her slowly expanding waist line she decides to look into enrolling in the nearby Silverman-Gym. There she runs into a girl from her grade named Souryuuin Akemi, who is also the Student Council President at her school. Akemi, who has a muscle fetish tries to get Hibiki to enroll in the Gym despite its high ratio of Macho-Men (which Akemi dislikes). Thankfully a beautiful Trainer named Machio appears and unknowingly convinces her to enroll and start her quest to a great body.