Hmph.  Let me see if I can remember the cheezy layman's explanation that
is not to be taken seriously.  =)

Farmer Bob has 13 daughters.  12 of these daughters have smallish chests,, and
      all else being the same, they have about a 50% chance of getting
      hitched to some good old boy.
But daughter 13 had a wierd mutation, and thus has larger glands, which is
      so appealing to the menfolk that she's got a 90% chance of
      reproducing.
So daughter 13 gets married, and so do half of her sisters, and they
      each have 10 daughters each (imagine the family reunions!)  Daughter
      13's eyes carry through to all ten of her daughters.  The other 6
      ( the sunken) daughters pass on the trait to their 60 children.
So 9 of the 10 elephantitis-affected daughters will then proceed to
        snare husbands and have 10 daughters a piece.  However, only 30 of the
      60 less hefty women will be able to appeal to the brainless
      favorabilities of men, and thus also get husbands.  They also have
      10 daughters a pop.
Out of 90 greatly-endowed women, 81 will get hitched and have 10 kids.
      Out of 300, only 150 feebly equipped women will get lucky and
      have 10 kids.
Out of 810 unbalanced women, 729 will reproduce 10 daughters.  Out of 1500
      unendowed women, only 750 will produce 10 daughters.

Needless to say in the next generation, large breasted women will vastly
outnumber small-breasted women.  Why?  Because it greatly enhances their
reproduction rate.

The model IS crude, sexist, and simplistic, but do you get the point?

Or do we have to demonstrate with penis size?

--Heretic, who offers any feminists out there a coupla grains of salt...