I charted the semen volume to body weight ratio (in mL per kg) for various species and honestly no particular pattern is emerging here.

("Electro" means that the specimen was obtained via electroejaculation, which may produce less robust loads than natural stimulation, but I was unable to find a peer-reviewed paper from someone who had managed to jerk off a rat. They did jerk off a gorilla, though! The documentation includes a detailed description of the process of training a gorilla to allow a handler to perform "penile massage" through a fence. It even mentions that only the thumb and forefinger were used. "Hung like a gorilla" ain't much.)

I went to one of those plasticized bodies exhibits at the Museum of Natural History in Vienna, except it was for animal bodies, not humans. Anyway, there was a maybe-8th-grade class there on a tour, and I figured that my German, though fluent, was rusty enough to benefit from an 8th-grade level explanation of what I was looking at, so I tagged along.

Toward the end of the exhibit, they got to the single human in the whole show: a (naked) human male, on display next to a (naked) gorilla of similar height. “So, what do you notice about these two?” asks the person giving the tour. The kids chime in with a bunch of obvious differences — the gorilla is more muscled, substantially heavier, has different limb proportions, etc. At the end, the guide says quite matter-of-factly, “so the gorilla is bigger for its height, and stronger, but of course it’s obvious that its dick is a lot smaller, as are its balls. Why is that?” After a little embarrassed muttering by the group, she went on to give some explanation about how mating fitness is signaled in gorillas via behavioural dominance, so dick size doesn’t matter much to them; once it was clear it was going to be pure speculative evo-psych bullshit I kind of tuned it out.

In the final room they had a plasticized giraffe and an elephant. That was cool to see.