A girl’s breasts may be a small one or a large one due to genetic factors, developmental reasons, tumor reasons and so on. 1. Genetic factors: the human organ is affected by genetic factors, there may be asymmetry between the two sides, a large and small situation, the general difference is not large. 2. Pubertal development: if the development process is affected by various factors, such as developmental asynchrony, there may be a large breast and a small one, and the difference will be reduced after development and maturity. 3. Tumor factors: when a tumor appears in the female breast, one side of the breast will be enlarged, and a lump can be found by touching, sometimes accompanied by lactation or orange peel symptoms. A girl’s breast may also be traumatic inflammation and other effects, breastfeeding during breastfeeding incorrectly and other reasons, you need to go to the hospital in a timely manner, to exclude the factors of disease.