One big breast and one small breast may be caused by growth and development, or it may be caused by breast cystic hyperplasia, breast sarcoma, breast cancer and other diseases, which need to be analyzed according to the specific situation. 1. Growth and development: If the breasts are of different sizes during the growth and development period, but there is no pain, swelling and other uncomfortable feelings, it is usually a normal situation. As the breasts have different sensitivity to growth hormone, estrogen, etc., the breasts may appear to be small or large. Generally, no treatment is needed, and the breasts will tend to be symmetrical as they mature. 2. Breast cystic hyperplasia: Breast cystic hyperplasia may be caused by the imbalance of the ratio of estrogen and progesterone, which results in excessive proliferation of the breast parenchyma as well as incomplete recuperation, and the main symptoms are breast swelling and pain, lumps, etc., which may result in the appearance of a small breast. 3. Breast sarcoma: Breast sarcoma is one of the rare malignant tumors, often manifested as a breast lump, which may be large in size, but has obvious boundaries, good mobility, and dilated veins visible on the skin surface. As a result of breast lump, it may lead to the appearance of a large and small breast. 4. Breast cancer: Breast cancer is also one of the malignant tumors, the early manifestation is the appearance of painless, solitary small lumps in the diseased side of the breast, the lumps are hard, the surface is not smooth, the demarcation with the surrounding tissues is not very clear, and it is not easy to be pushed in the breast. With the enlargement of the tumor, it can cause localized bulging of the breast. Therefore, it is possible to have a large breast and a small breast. If physical symptoms are excluded, it is recommended to go to the hospital in time for treatment.