Sexual Selection and Clitoris Size:
"Based on this analysis of the adaptive functions of female orgasm, we can start to understand why the female clitoris remained small and relatively hard to stimulate, while the male penis enlarged to unprecedented proportions in our linage. Maintenance of a small, relatively hidden, and relatively hard-to-stimulate clitoris enhanced female discernment and choosiness with respect to assessing sexual stimulation by males. A larger, more protuberant clitoris which could have been effectively stimulated by almost any tactile contact, however inept, would have made its female bearer less able to judge the relative stimulation capabilities of different males. She might have mated with less sexually competent males and produced less sexually competent sons."
(Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 215)