In a recent study in Biology Letters, Vahed and colleagues measured the size of testes in a number of bushcricket species. They found that in species with slutty females (i.e., a high level of polyandry), the males have larger testes. Somewhat surprisingly, however, larger testes do not lead to larger ejaculations, but instead to more frequent ejaculations. Clearly, the males are quite promiscuous as well.
Source: Wikimedia Commons
However, the real reason for mentioning this study is that Vahed and colleagues found a particular species of bushcricket, in which the testes account for a whopping 13.8% of the males' total body weight! The most ballsy animal ever found.
Vahed, K., Parker, D., & Gilbert, J. (in press) Larger testes are associated with a higher level of polyandry, but a smaller ejaculate volume, across bushcricket species (Tettigoniidae). Biology Letters. doi: 10.1098/rsbl.2010.0840. [Link]