One of the most common questions that teenagers ask me is how big their breast size is going to be. If their friends are growing rapidly and they are still somewhat flat chested, for obvious reason, many girls get worried if they will develop breasts and how large will they get. I have typically told them that the final size is a combination of genes and diet. For instance, if women in your immediate family have normal size chests, that is what you should expect, as long as you eat a healthy diet but there is no guarantee, like most things about you (BTW, your race is not guaranteed even if both your parents are of the same race because as we are finding out through many television programs, so many people worldwide are not as pure as they thought).
So as far as the genetic factors are concerned, research completed by Nicholas Eriksson for 23andMe has been able to identify seven genes that are linked to chest size. So if you have more of these genes or they are more dominant, then be prepared to show off your cleavage.
There is some bad news, though. It has also been found that the genes that impact the size of the chest are also linked to breast cancer. In any case, just let Nature do its job while you live a healthy lifestyle.