Small testicles may be a normal phenomenon, but may also be caused by androgen deficiency, orchitis, varicocele and other factors. 1. Normal phenomenon: if the boy has not yet entered puberty, due to the reproductive system is not fully developed, the testicles may be relatively small compared to adults, which is usually a normal phenomenon, after entering puberty, the testicles will gradually develop. 2. Lack of androgen in the body: When boys are in puberty, if there is a lack of androgen in the body, it may affect the development of testicles, resulting in underdevelopment of testicles and penis. 3. Orchitis: If he suffers from bacterial orchitis or mumps orchitis, the necrosis of testicular tissue structure may lead to testicular atrophy, thus making the testicles very small. 4. Varicocele: it may be related to heredity, venous reflux disorder and other factors, which may affect the secretion of sex hormone level and lead to abnormal testicular development, which may make the testicles too small, and also lead to male infertility. To summarize, if the testicles are small, it is recommended to seek medical examination and treatment to avoid serious effects.