A 5 year old child with one extra large testicle and one small testicle may be caused by factors such as syringomyelia, acute injury and orchitis. 1. Syringomyelia: it may be related to infection, injury and other factors, which may cause the amount of fluid in the syringomyelia cavity to gradually increase, which will lead to one testicle being particularly large and the other being smaller, and may be accompanied by scrotal distension. 2. Acute injury: when children are engaged in some strenuous exercise, the testicle area suffers from acute injury, which may cause the outer testicle to swell rapidly, and then the testicle will be particularly large, accompanied by severe pain and redness and other symptoms. 3. Orchitis: For example, acute mumps orchitis, usually associated with mumps virus infection, can lead to localized damage to the testicles, causing testicular swelling, which can lead to one side of the testicle is particularly large, accompanied by testicular pain, falling, redness and other symptoms. A 5 year old child with one extra large testicle and one small testicle may also be caused by other factors, it is recommended to consult a doctor in time for the appropriate examination and treatment to avoid causing serious complications.