Both brain tissue and sperm cells require a lot of metabolic energy to produce and maintain. The different species appear to have evolved a preference for developing one organ more than the other, presumably determined by which will help them produce more offspring.
“An extraordinary range of testes mass was documented across bat species - from 0.12% to 8.4% of body mass. That exceeds the range of any other mammalian order,” says Scott Pitnick, from Syracuse University in New York, US, one of the research team. Primate testes vary between species from 0.02% and 0.75% of body mass.
Apparently, then, you can have a large brain (such as in humans) or large testes (such as in some bats and chimps) but not both.
If I remember corectly, one of the areas where researchers recently found differences between chimps and humans was in the genes for sperm production. So a change in mating patterns and social structure between chimps and humans may be one, of several, forces driving the evolution of large brains in humans.