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In mammals with external testes it is most common for one testicle to hang lower than the other. It is estimated that in about 65% of men the lower hanging testicle is the left one Scrotal Asymmetry: Right-Left and the scrotum in male sculpture" [1] " By I. C. Manus . This is due to differences in the vascular anatomical structure on the right and left sides.

Short of direct injury or subjecting them to adverse conditions, e.g., higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Steroids taken for muscle enhancement often have the undesired side effect of testicular shrinkage. Similarly, stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testes may shrink or atrophy during hormone replacement therapy.

There are a number of mechanisms to maintain the testes at the optimum temperature.

At about week 6, sex cords develop within the forming testes. These are comprised of early Sertoli cells that surround and nurture the germ cells that migrate into the gonads shortly before sex determination begins. In males, the sex-specific gene SRY that is found on the Y-chromosome initiates sex determination by downstream regulation of sex-determining factors, (such as GATA4, SOX9 and AMH), which leads to development of the male phenotype, including directing development of the early bipotential gonad down the male path of development.

Only the Boreoeutherian land mammals, the large group of mammals that includes humans, have externalized testes. Indeed their testes function best at temperatures lower than their core body temperature. Their testes are located outside of the body, suspended by the spermatic cord within the scrotum. The testes of the non-boreotherian mammals such as the monotremes, armadillos, sloths, elephants remain within the abdomen. http://www.uwyo.edu/wjm/Repro/spermat.htm There are also some Boreoeutherian mammals with internal testes, such as the rhinoceros.

It is argued that those mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes.

This position is made less parsimonious by the fact that the kangaroo, a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, have also been subject to heavy sperm competition and thus developed external testes, however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.

Source: Wikipedia > Testicle