ANDY'S ANSWER: Breasts are made up mostly of fatty tissue and how big your breast get depends on how much fat your genetics dictate that you will store there. If your mother is big-busted, there's a good chance that you also will be, but if she is flat-chested, you could also end up with smaller breasts. If you gain weight over your whole body, your breast will also gain weight, but I don't think that was what you had in mind.