Hypogonadism: This condition is characterized by the underproduction of sex hormones, which can lead to small, nondistended testicles. It is a common cause of testicular atrophy and can result from various factors, including genetic disorders, infections, and certain medical treatments.

Klinefelter Syndrome: A genetic condition in males where they are born with an extra X chromosome, leading to small testes, infertility, and other hormonal imbalances.
Testicular Atrophy: Can occur due to various reasons such as injury, infection (orchitis), or blockage of the blood supply to the testicles, leading to shrinkage of the testicles.
Cryptorchidism: A condition where one or both testicles have not moved down into the scrotum, which can lead to smaller, nondistended testicles if left untreated.

Testicular Cancer: Although testicular cancer often presents with a testicular mass or swelling, in some cases, especially in advanced stages, it can cause testicular atrophy, making the testicles feel smaller or nondistended.
Testicular Torsion: A medical emergency where the spermatic cord becomes twisted, cutting off the blood supply to the testicle. Early stages might not always present with the typical acute pain and swelling, making it crucial not to miss.

Myotonic Dystrophy: A rare genetic disorder that affects muscle tone and can lead to testicular atrophy among other symptoms.
Noonan Syndrome: A genetic disorder that can affect various parts of the body, including the testicles, potentially leading to smaller size.
Germ Cell Aplasia (Spermatogenic Arrest): A condition where there is a failure of the development of germ cells in the testes, leading to small, nondistended testicles due to the lack of sperm production.