The two buttocks are not the same size, and the smaller one has pain, to exclude the existence of long-term sedentary muscle injury or disc herniation, tumor problems. 1. Muscle strain caused by long-term sedentary: patients engaged in long-term office work for a long time sedentary, resulting in lactic acid deposition in the buttock muscles, so the problem of pain in the buttocks may occur, due to the poor sitting posture and resulting in the gluteal musculoskeletal fasciitis, can be manifested as a side of the buttock muscle pain. 2. Herniated disc: the herniated lumbar disc compresses the sciatic nerve and leads to different size of buttocks on both sides, which may be accompanied by pain or numbness in the lower limbs and pain when walking. 3. Tumor bone metastasis: it is also necessary to exclude whether there is pain caused by tumor metastasis, which will not only manifest as pain in buttocks and sciatic nerve, but also be accompanied by fever or weight loss. If you have just found that the two sides of your buttocks are not the same size and there is pain in the small buttocks, it is recommended to consult a regular hospital for diagnosis and treatment.