The game has something similar to what you describe in that girls can lift weights at the leisure centre and the higher their fitness percentage the more weight they can gain before becoming Barely Mobile or Immobile. But I could take this further.
I had a previous request for more muscle growth, and I was thinking about combining it with another suggestion for a cooking minigame.
As well as whipping up fattening meals & treats, perhaps there could be rare or special ingredients to create meals that induce other kinds of transformation. I'm envisaging a muscle growth food or drink that lets a girl break past the 100% fitness cap, perhaps increasing the cap to 200% and unlocking a further two muscle growth stages.
This could also be the vehicle for some other similar suggestions, such as height change.