The testicles are a very important organ of the male reproductive system. There are usually 2 testicles, located in the scrotum, and the size of the testicles usually varies little and is almost the same in volume. The size of a normal testicle is usually 4cm by 3cm by 2cm in size, which is about the size of a pigeon’s egg. If the testicles appear to be different sizes, it can be seen as an increase in the size of one testicle or a decrease in the size of one testicle. Testicular enlargement is commonly associated with testicular masses, such as testicular tumors, which tend to cause testicular enlargement and harden the testicle. Also seen is testicular syringomyelia, an increase in fluid in the testicular sheath cavity, which can also make one side of the testicle feel significantly larger. Another reason is the reduction in the size of the testicles. Various reasons cause one side of the testicle to atrophy, which also tends to cause the 2 testicles to be different in size, and the specific cause needs to be clarified through relevant examinations.