It's just a power play by the chick. Chicks know guys who look good and are on gear probably wouldn't look as good as if they weren't on the stuff, hence less chance the guy would cheat or be hit on by other women. So you get off the stuff then in the end the relationship doesn't work out and not only don't you have a girlfriend but your body looks like crap.
Chicks do tons of diet pills and things like ECA to stay thin, because that's the look that women stive for, and what is attractive to men. Men on the other hand like to be big and lean, feel good when they're this way, and this is attractive to women. Funny how society takes all the drugs that are good for achieving that look and locks them up somewhere, and we're left injecting something out of a bottle with a picture of farm animals on the label.
Tell her straight up you like how you look and feel just fine and it's your body to do what you want with, and if she doesn't like it to go find some scrawny dude who's favorite machine at the gym says "stairmaster" on the side.