Why is one testicle generally larger in humans? Just due to asymmetric development, or is there a hypothesis as to some utility it serves? I read that the different heights may be attributed to attempting to keep them from hitting against one another. Is there a similar guess to the purpose for the difference in sizes?
I would strongly question whether the underlying claim that testicular asymmetry is generally true. While there is no known clinical significance of testicular asymmetry, most men probably have no noticable difference in testicular size. There have been a number of studies over several decades, and findings have been inconsistent.
Braus (1956) stated that the left testis was heavier and lower; Chang et al. (1960) determined on study of cadavers that the right testis was generally larger than the left.
A 2008 study in 0 to 6-year-old boys found no difference in testicular volume measured ultrasonographically.
Age or level of sexual activity may also play a role; it is known that testes get larger during sexual activity.
The reason for testicular asymmetry is unknown. It has been reported in many different species (see Yu, 1998). The most common explanation provided in humans is that the right testis is more commonly larger than the left due to a difference in vascular anatomy: the left testicular vein empties into the renal vein while the right testicular vein empties into the inferior vena cava. However, anatomy alone would not explain testicular asymmetry in other species as well, so there is no doubt more to this than presently understood.