The different sizes of breasts during breastfeeding are considered to be caused by breastfeeding habits, uneven distribution of estrogen, mastitis, etc. They can be treated by sucking more on the smaller side of the breast and taking penicillin. 1. Breastfeeding habits: Breastfeeding is often done with one side of the breast, the baby long time partial milk will lead to breastfeeding two sides of the breast size is not the same situation. This can be improved by letting the baby suckle more on the smaller side of the breast. 2. Uneven distribution of estrogen and progesterone: During pregnancy and breastfeeding, the secretion of estrogen and progesterone increases, and the breast buds of the more sensitive side of the breasts will be developed in priority, which will lead to the situation of different sizes of both breasts during breastfeeding. If there is no discomfort, there is no need for treatment. 3. Mastitis: Mastitis can be caused by the accumulation of milk and bacterial infections, which can lead to different sizes of breasts during breastfeeding. You can take the breast milk and take antibiotics such as penicillin and cephradine to treat it. Breast tumors as well as breast hyperplasia should also be considered as the two sides of breasts are not the same size during breastfeeding. When the two sides of breasts are not the same size during breastfeeding, go to the hospital in time to check whether there is a cause of the disease, and carry out the treatment according to the result. Medications should be applied under the guidance of a doctor.