Actually, the biggest factor in determining the size is genetics. If you come from a family of women who have large breasts, you are more likely to be large-breasted. But remember, that's just an average. There are plenty of small-breasted women with moms who have large breasts. Besides, there is no normal breast size.
Society puts too much pressure on women to have perfect breasts.
The fact is, whatever size you are is the perfect one for you. Don't get all hung up with comparing yourself to your friends. There are advantages to having either large or small breasts as long as you are happy with who you are. So whether you are small, super curvy, or somewhere in between, make the most of your body. Unfortunately, we live in a world which can sometimes encourage us to be overly concerned with our bodies.