I read — Lord forgive my search history — that testicle size corresponds with animal fertility.
It’s more closely related to the female propensity to shag around in that species. Being able to drown out the sperm of male rivals works. Of course, this is also relative to body size, but gorillas have smaller than bonobos, with humans in between.
The classic case of this being Soay sheep. Similar, at one point, to mainland cousins, they’ve been wild for centuries now. Which means that the rams haven’t been culled as wethers etc. The competition for access to ewes has meant the rams that do successfully procreate are those with the Buster Gonads…….an inheritable condition.
Oh, and it’s also possible to run this back the other way. Testicle size is a good guide to the long run female propensity to shag around in that species…..