One large testicle and one small testicle refers to the asymmetry of the testicles on both sides, which may be a normal physiological phenomenon, or it may be caused by certain diseases, such as varicocele, testicular sheath effusion, etc., which should be consulted in time to find out the cause of the disease. 1. Physiological phenomenon: normal men have two testicles, located on the left and right sides of the scrotum, which are usually a large one and a small one, a high one and a low one, not completely symmetrical, related to the development of testicles, which is a normal phenomenon. However, this difference is small and will not affect the normal physiological function of testicles. 2. Pathological factors: (1) Varicocele: due to the obstacle of reflux of spermatic vein, resulting in local venous dilatation and tortuosity, the patient may have the discomfort of heavy scrotum and crushing pain, etc. Varicocele can also affect the level of sex hormone secretion, resulting in abnormal testicular development, atrophy or a large and a small symptom. (2) Testicular syringomyelia: excessive accumulation of fluid in the testicular syringomyelia cavity can be spherical or pear-shaped cystic swelling. When the accumulation of fluid increases, the size of the testicle can become bigger, and the symptom of one big and one small testicle appears, and the patient can be accompanied by drooping and swelling of the scrotum, and it can also affect urination, walking, sexual life, and so on. In addition, testicular inflammation, testicular tumor, hernia and other diseases can also lead to the symptoms of a large testicle, but this symptom alone is not enough to diagnose the disease, it is recommended that the patient consult the doctor in a timely manner, improve the relevant examination, the doctor to make a clear diagnosis, and then carry out the appropriate treatment.