Testicle size and reproductive potency are entirely unrelated to penis size, for all the men out there. And if you only ever have one partner at a time that you want to make babies with, you don’t really need to be a super stud in the testicle department. You just need regular sexual contact with your lady half.
But this shows that even if you do want all the ladies, large testicles don’t matter.
(Aside; people with breeder fetishes who impregnate multiple women are fucking gross and weird. Musk is one such and just.. eew.)
In primates small testicle size is correlated with how many males a female typically mates with. More males mating with the same female means the male that produces the most sperm has an advantage. So gorillas have tiny balls because every troop only has one silverback male, and chimps have horrifically huge nuts because there’s a lot of competition for mates from other males in the troop.
Interestingly, humans fall roughly in the middle of the scale in terms of ball size!