Under normal circumstances, bilateral breasts are two symmetrical breasts arranged between the ribs of the second to sixth ribs in front of a woman’s chest. If one side is large and the other is small, the possible reasons are as follows: 1, congenital reasons, some women may have incomplete development of the chest wall on one side, and the chest wall is absent, such as patients with Poland syndrome can have chest wall absent performance, at this time the breast will look smaller, the breast after the pectoral muscle and rib cage did not hold it up; 2, acquired reasons, there are two cases, one is the presence of tumors on one side of the breast. After the tumor grows, the breast looks bigger than the opposite side, which is generally common in young girls and belongs to developmental asymmetry; secondly, excessive breastfeeding on one side and no breastfeeding on the opposite side, resulting in bilateral breast asymmetry, which is caused by late breastfeeding.