people in general are bigger, so i dont understand why the birth canal wouldnt enlarge accordingly. i think its more about women wanting to keep their vag tight. since medical capabilities can now allow them to do this with the safe c-section procedure, and combined with the fact women are going to further and further extremes to get stay vain and competitive with each other (more single mothers, still need to date, self concious, etc.) they find this the better option