It circles around fighting sperm:

https://www.realclearscience.com/blog/2024/02/03/why_gorillas_have_tiny_testicles_and_s...

"Is there anything more quintessentially masculine than a silverback gorilla? Broad and barrel-chested, with sculpted arms and legs as thick as tree trunks, these adult males are imposing sights. But a quick glance between their legs greatly diminishes the awe...

"That's because male gorilla testicles weigh a mere 30 to 35 grams, positively puny compared to their hulking 350-pound frames (159kg). And their erect members flex just two and a half inches.

"At less than half the body weight of gorillas, chimpanzees sport testicles weighing five times more! And the chimp penis can be as long as an average human's (a little over five inches).

"So where do human men stack up in this "pissing" battle between primate cousins? Much closer to gorillas. The testicles of adult men tip the scales at about 20 grams. Relative to body weight, that's three times larger than gorillas' but five times smaller than chimpanzees'.

"Mating strategy explains this drastic variation in reproductive organ size. Gorillas are polygynous. In gorilla troops, one male – the silverback – physically outcompetes all others to earn mating rights with the females. In chimpanzee troops, on the other hand, males and females mate with multiple partners in a game of sexual subterfuge. The competition for fatherhood thus takes place inside females' reproductive tracts, between the males' sperm. Chimps' massive testicles churn out ejaculate that contains 14 times more sperm than gorillas'. A male needs an army of swimmers if he wants to have any chance of siring offspring.

"Over humans' evolutionary history, we have tended to practice either polygyny (like gorillas) or monogamy. In both mating styles, reproductive competition takes place outside the body rather than inside, meaning that males don't need the sort of sexual machinery seen amongst chimpanzees."

Comment: battling sperm in chimps!! Bonobos are very promiscuous. I wonder about their size.