The causes of small breasts in girls mainly include genetic factors, endocrine factors, malnutrition and so on. 1. Genetic factors: girls with small breasts and genetic factors have a certain relationship, the mother of small breasts may also be inherited by the daughter, but will not be 100% hereditary. 2. Endocrine factors: breast development and estrogen, if the level of estrogen is relatively low, it will affect breast development, resulting in girls with small breasts. 3. Nutritional deficiencies: During the period of breast development, if the nutritional intake is not sufficient, it will affect the normal development of breasts, which will lead to small breasts in girls. There are many reasons why girls have small breasts, and they may not simply include the above. However, girls with small breasts will not affect the health of the body, it is not recommended to abuse the method of breast enlargement, so as not to cause adverse effects on the body. You can also go to the hospital, the professional doctor to give an assessment to find the cause, targeted treatment.