The reasons why the two sides of the breasts are not the same size are as follows: 1, congenital development, anyone’s left and right side of the breast size is not exactly the same, there will be some slight differences, will not significantly affect the aesthetic appearance of the breasts, and there is no pain, itching, and other symptoms. 2, and sports, labor, etc., long-term use of the upper limbs of the side of the physical labor or sports, the side of the breast may be slightly larger than the opposite breast. 3, the influence of disease, suffering from breast fibroma, breast tissue hyperplasia and other female diseases, the diseased side of the breast may be significantly larger than the opposite healthy breasts.