6-year-old boy testicles are not the same size, usually refers to a large testicle a small, may be a normal physiological phenomenon, may also be caused by certain diseases, such as testicular inflammation, testicular syringomyelia, testicular tumors, etc., should be timely consultation, to identify the cause of the disease. 1. Physiological phenomenon: testicles are male reproductive organs, located in the scrotum, normal pediatric testicles are located in a high and low position, the size of the relatively large, but there will be slight differences. 2. Pathological factors: (1) Orchitis: when bacterial or viral infection causes testicular inflammation, the patient’s testicles may be enlarged or atrophied, resulting in a large testicle and a small testicle, accompanied by redness and swelling of the scrotum on the affected side, pressure and pain, and systemic symptoms such as fever, chills, nausea and so on may also occur. (2) Testicular syringomyelia: it occurs in children and refers to the excessive accumulation of fluid in the testicular syringomyelia cavity. There are no obvious symptoms when the accumulation of fluid is small, but when the fluid accumulates to a certain extent, it may cause symptoms such as scrotal swelling, scrotal enlargement and urinary abnormality. (3) Testicular tumor: refers to the tumor occurring in male testicles, usually malignant lesions, which may also lead to a large and small testicle on both sides, and may also appear scrotal mass, dull pain in the scrotum, lower abdominal distension and other symptoms. A 6-year-old boy with a large testicle should seek prompt medical attention, and be further judged by the doctor based on the child’s medical history, physical examination, auxiliary tests, etc., and treated under the guidance of a physician if necessary.