I enjoyed the demo, and I hope you continue development. Glad to see more AE representation. I don't know what notes to give you with how little there is and without knowing the scale or scope of the game, so take the following as mere suggestions.
If you want to do progressive expansion, I would suggest cutting the initial growth in half or even a third to give the player more to look forward to. You said you wanted to make them twice their on screen size, so this would give 4-6 stages for each character.
An unlockable gallery to see each girl's before and after is always nice to see in these types of games.
Mini games related to each girl's sport/excercise? The formula is meant to help athletes recover, so maybe this would be a good excuse for them to take it. Maybe there's some hesitancy for some girls to take the formula, so getting up to a certain score would make them exhausted enough to give your formula a try.
Alternatively, you could just choose to watch each girl do their exercises, and that's when the player has the option to give them the formula.
character archetype: I've always been a fan of confident characters who hype themselves up far above what they're capable of. Not necessarily brats, but people who believe they are the kings/queens of their own world despite how much real life constantly proves them wrong. I'm thinking for this story, a trash talking amateur/backyard mma fighter that is on a 0-7 losing streak and needs your help to train.
I hope you find these helpful. Don't overwhelm yourself. Try to pace yourself to keep your energy and interest up. You've done good with the demo.