The testicles are usually big on one side and small on the other, mainly in the following cases: 1. testicular sphingomyelitis, which refers to a large amount of fluid gathered in the testicular sphincter cavity, manifesting as an enlarged scrotum on one side and an indistinct testicle to the touch, which can be clearly diagnosed by scrotal ultrasound. 2. acute epididymitis, which is an acute bacterial infection of the epididymis, mainly manifesting as an enlarged epididymis and testicle on the affected side, with local skin redness and tenderness, which can cause a large testicle on one side and a small one on the other. 3. The testicular atrophy can be caused by trauma and testicular inflammation, which can lead to testicular atrophy, manifesting as large testicles on one side and small testicles on the other.