Fitness does that!
When your girls are above 50% fitness the descriptions for them change and there is quite some muscle in it! It also allows them to jog even at higher weights. Fitness further affects actions: gym, swimming, jogging, going for a walk and activities which take place in the bedroom!
... Status changes too to reflect the Fitness Level
read above - there is much more to it then just the number!
There is a lot of Words describing how muscular the girls are, and many activities are affected by the fitness level.
Saying this because i like this aspect of the game very much.
Ofc more of the above is welcome, but there is a lot of it already in there!
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.
Short Critic about "Height Change"
For realistic adult characters height change is not comprehensible and thus immersion breaking. There is a difference between exaggerating body changes that do exist in real life AND/OR making up completely fictional body changes like growing wings, horns or changing height.
The latter only fit in the fantasy-magical-fiction realm.
I hope purely fictional or magical features will be optional.
Looking forward to the update.
regards,