It is not unusual for one breast to be larger than the other. Breast asymmetry is associated with rapid growth spurts prior to and during puberty; that is, it occurs between the ages of 12 and 18. It is normal for one breast to respond more to surging estrogen levels. Large amounts of estrogen are released in cycles, and disruptions in the cycles can cause the breasts to grow differently. Breast asymmetry is a very common occurrence, especially among adolescent women who are still going through puberty. Asymmetrical breasts often correct themselves, but it is important to realize that most women's breasts are not completely the same. Almost every woman with natural breasts has a slight difference between the two. If your asymmetrical breasts are more than slight, such as a full cup-size or more in difference, I recommend a consultation with a doctor
who will first evaluate if there is any cancerous tissue responsible for the asymmetrical growth. You don't state your age. If you are a teenager, the doctor should advise you that, often, the smaller breast catches up in size with the larger one later in development.