Small breasts may be caused by congenital genetic factors, nutritional factors, endocrine factors and so on. 1. Inborn genetic factors: mainly due to genetic inheritance in the maternal or paternal line, the mammary glands themselves are relatively small, and the proliferation of the glands is not obvious, which will lead to a smaller breast volume. 2. Nutritional factors: mostly seen in thin people, may be due to the usual nutritional deficiencies, resulting in low body fat content, thus causing small breasts. 3. Endocrine factors: may be due to endocrine disorders in the body during breast development, estrogen levels are relatively low, which will lead to small breasts breast development is affected. Small breasts may also be due to incorrect wearing of underwear, chronic diseases and other reasons, you can also go to the hospital to clarify the cause.