This is an interesting question that is common, but demands more information before a complete answer can be given. Before discussing what is the specific cause of your small breasts, it is important to understand what you mean by "small." If you mean that they are smaller than your siblings, but otherwise normal, then it is more likely to be a physiologic variation. If they are fundamentally different, it could be a medical cause. Other questions that are important to understand revolve around your menstrual cycle and whether or not that has been regular, as well as the other sexual characteristics that women have such as the widened hips and pubic hair distribution.
If all of these other facts are completely normal, then it could also have something to do with your activity level and how thin you are. Thin women who are very active will often have smaller breasts than their larger counterparts. Obviously, this entire discussion is a difficult one to have without speaking to someone in person. Your OB/GYN would be an appropriate resource to discuss this matter with, and your next physical exam and preventive health screening (including breast exam and pap smear) is a great time to ask these questions.