One leg is thick, one leg is thin, the common reasons are the following: First, the physiological reasons, normal people two leg thickness is the same, but there are a few people congenital is not the same, belong to the congenital development of the problem. If there is no any obstacle to movement, this situation can be observed without treatment. Second, lower limb vein thrombosis, vein thrombosis will make the leg blood flow obstruction, bruising and swelling will appear thickening phenomenon, common in patients who are bedridden for a long time, do a lower limb vein ultrasound can basically confirm the existence of this condition. Venous evidence of lower extremity needs to be treated immediately, because it is dangerous to cause pulmonary embolism after the embolus is dislodged. Thirdly, trauma, after trauma, the legs can’t bear weight bilaterally, which can lead to muscle wasting atrophy and make the legs thinner, which can promote recovery through rehabilitation exercises. Fourth, the iliac vein, femoral vein compression, some reasons lead to large vein compression will cause venous reflux obstruction, will also appear on one side of the leg thickening symptoms. In short, if you find that the legs are not as thick, you should go to the hospital in time to find out the cause.