Breasts are composed of muscle and fat. You can't do much about the muscle and anyway the muscle isn't the problem. The fat is what takes up the space. Unfortunately, there isn't any way to reduce the fat in just the breasts alone. As so many women have discovered to their dismay, breast fat is often the first to go when they diet. That's just the way it is. You rarely (outside of Hollywood :)) see a very thin woman with huge breasts.