I'd I get healthier, will my boobs get smaller?

I'm a 34upperC-lowerD, and I weigh about 120-130 lbs. I'm a vegan, but I still eat a ton of junk. I don't want to lose weight, because I recently gained 20lbs that I needed. If I eat a lot healthier and start working out, how can I do it without losing weight or my boobs getting smaller?