Для мальчиков — большой пенис как причина прямохождения:

This unique adaptation is thought to have been selected through female mate choice, and by the time Homo erectus arrived on the scene, the hominid penis was significantly longer, fatter and more bendy than our ape cousins’. It has even been theorised that bipedalism evolved in humans to allow the fashionably new, larger, flexible penis to be displayed to discerning females.

http://www.guardian.co.uk/science/2010/may/06/women-penis-size

И для девочек — большая грудь как милое украшение:

So it has been widely theorised that the plump buttock and bosom of modern women are sexual ornaments, selected for by ancestral males. Seen from a distance the adult female form, either from behind or from the front, can be recognised as distinct from the male of the species. An hourglass figure, plus youthfulness, would have attracted male hominids looking for mating opportunity.

http://www.guardian.co.uk/science/2010/may/14/breast-size-evolution

Хотя, должен заметить, что в последней статье тема размера определенно не раскрыта.