Sorry, no picture with this one.

It turns out that size does matter, at least to some species.   Males of animal species in which females are more promiscuous tend to have larger testicles.   They also tend to have smaller brains.   Scientists postulate that this is because in order to pass on your genes, when you are competing with your female's many other love interests, you must have more sperm than your competitors.   In "successful" members of such species, more energy goes into making bigger testicles and less goes into brain development.