Most women will lost some of their breast size when they lose weight. The fat reduction generally comes from all parts of the body, including the bust. Though some may lose more from it than others, as it does have a bit to do with genetics.
as far as exercises, you cant do anything to specifically reduce the size of the bust, but you can do some things to help alleviate some of the issues that women with larger breasts tend to have. Most specifically, working the pectoral muscles (chest muscles underneath the breast running up to the shoulder) can help keep the breasts looking more perky and less slouchy. Standard resistance based strength training should be able to accomplish this.
