A big gonopodium can be a drag.

Male mosquitofish with bigger sex organs get more tail, but also tend to get eaten.

A biologist at Washington University in St. Louis has shown that for some fish species, females prefer males with larger sexual organs, and actually choose them for mating. That does not exclude males with an average-sized sex organ, called a gonopodium. These fish out-compete the larger-endowed males in a predator-laden environment because they have a faster burst speed than the males with larger genitalia, who lose out because the size of their organ slows them down, making them ripe for capture by larger fish.

In grade school, I was a slow runner and got beaten up a lot. Just sayin'.