Factors that cause the breasts on both sides to be not the same size include physiological and pathological, physiological if the difference is obvious needs to be treated, and pathological can be treated by surgery. Normally, most people’s breasts are not exactly the same size. This phenomenon is most common during the growth and development period, when both sides of the breasts do not have the same degree of sensitivity to hormones, resulting in the appearance of different sizes. Usually, no special treatment is needed, and the breasts will gradually become symmetrical as they grow and develop. If the difference is significant, treatment is needed. Some pathological factors may also cause the two breasts to be different sizes, such as unilateral breast fibroma, unilateral breast cancer and other unilateral breast-occupying diseases. Due to the presence of a mass on one side of the chest, it appears larger than the opposite side. This condition usually requires surgical treatment to remove the mass. Under normal circumstances, breasts that are not the same size do not affect the health of the organism and it is recommended to look at it with a normal mindset.