Is there a way to reduce your bust size by exercise?
It's hard to. Female long distance runners, really dedicated ones, often run off all their excess body fat and this can lead to shrinking of the breast tissue. But just doing a good workout 3 or 4 times a week will not decrease your bust size to any extent that you'll ever see. For women with health problems from large breasts (back problems, etc.) breast reduction surgery is a viable option.