One large testicle and one small testicle may be a normal physiological phenomenon, or it may be caused by testicular inflammation on one side, or it may be caused by testicular syringomyelia on one side. 1. Physiological factors: the slight difference in the size of testicles on both sides may be related to the different developmental speed of testicles on both sides, if the volume of testicles on both sides is between 12~25ml, and the difference in the size of testicles on both sides is less than 20%, then it is generally a normal phenomenon. 2. Unilateral orchitis: unilateral orchitis is a kind of disease caused by bacterial or viral infection, which can lead to the symptoms of one testicle being big and the other being small, and the patients will also have the symptoms of redness and swelling of the scrotum on the affected side, testicular pressure and pain, fever, chills, nausea and vomiting. 3. Unilateral testicular syringomyelia: Unilateral testicular syringomyelia can also lead to the symptoms of one testicle being big and the other small, and the patient may also have painless cysts in the scrotum or groin, accompanied by a feeling of drooping, etc. The above symptoms may also be caused by other factors. The above symptoms may also be caused by other factors, after excluding physiological factors, if the symptoms of one testicle big and one testicle small persist, it is recommended to go to the regular hospital for timely consultation and treatment, in order to strive for early recovery.