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.
The only suggestion I have would be to work your chest or pectoral muscles with exercise to make your breasts appear firmer and perhaps that will make them look larger. Eating healthy is a great idea for your whole body, and for sure will give you the best chance for development. Other than that, I think you'll just have to wait it out like everyone else.