It is not unusual for breast growth to start on one side before the other
Physical Development in Girls: What to Expect During Puberty Unequal breast size
It is quite common for one breast to be bigger than the other as development occurs during puberty. Usually the breasts become the same size over time and do not need any treatment. However, if the breasts have not become more or less an equal size by the age of about 16 years old (or near the end of puberty), they will probably remain unequal. About one in four adult women have some degree of asymmetry of the breasts.
In some adolescents the problem is that one breast is overdeveloped, whereas in others the smaller breast is underdeveloped.
When to see a doctor
There is no need to see a doctor for unequal breast size during puberty (when the breasts are still developing). However, you may wish to see a GP for advice if the breasts are of unequal size at the end of puberty. You may be referred to an endocrinologist (doctor specialising in conditions related to hormones).
Kids Health Information : Unequal breast size
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