Anything as long as it's something of a natural part of her that she can't or won't get rid of
Cow girls who get bigger if they milk themselves too little or too much is one idea, with bodies stronger than normal humans simply to be able to heft around the weight of their inhuman chests
Don't like it too much when it's something forced on her by someone else specifically though. Either it should be something she's getting naturally or something she's willing to embrace

Soon™