It depends on how large they are already. In women they are mostly fatty tissue. Bigger ones have more fatty tissue than small ones. If they are big they have more fat to lose - the muscles and glands don't shrink from losing weight, the fat does. The skin takes a long time to shrink so expect some severe sagging if you lose a lot of weight. Implants are not good for your health and people have had serious problems from them. If you do lose a lot of weight and want to have some kind of cosmetic surgery to hide the sagging I think just getting a lift (tighten the skin a little) is much safer. Personally, I don't like the idea of cosmetic surgery.