I work out every day in the university gym, where in the shower room it is obvious that the large more macho males subtly (or not so subtly) check out the 'packages' of their shower mates. Here's the payoff in another species, illustrated by Kahn et al.'s study showing that female mate preference in mosquitofish is influenced by size of male genitalia. The tested female preference for males that had had their genitalia considerably reduced in size by surgery compared with those with only a minor reduction (see figure). They found that females spent, on average, around one-and-a-half times longer associating with the better-endowed males. This preference was, however, only expressed when females chose between two large males; for small males, there was no effect of genital size on female association time.
Representative G. holbrooki showing the difference between wild-type (not used in trials) and treatment male gonopodia. (a) Wild-type; (b) minor reduction; (c) major reduction. Scale bar 1 mm.