I think it's because it's assumed that a woman who has a 'too large' clit is assumed to be intersexed in some way. Even if, in fact, there is nothing hormonally/biologically different than any other woman. Since, from what I've seen (almost exclusively based on porn images of woman's genitalia so there's a large grain of salt here), woman's genitals vary pretty widely, I'm really not sure how 'too large' is determined anyway.