Breast tissue is largely adipose or fat. There is an amount of glandular tissue, relevant for lactation, but largely breasts are composed of fat. Breast size is governed by body weight fat proportions, gravity and also by genetics. If your Mum had big boobs when she was thin it is likely that you will too. However, if you are desperate to reduce your breast size, you will definitely see a reduction if you reduce weight SENSIBLY. Remember that as breasts are largely composed of fat crash dieting may severely damage the shape of your breasts, making them droop or leaving them looking like a tennis ball in a sock (something that often happens when all the fat is depleted leaving the skin and glandular tissue).Please don't be disheartened. If the worst comes to the worst, there are excellent supportive bras (and not all of them look as if they were designed to supplement support onthe Golden Gate Bridge) and as a very last resort - surgery (not for everyone, I know).