If an organism is evolving in limited resources, it slowly changes to make the best & most efficient use of those resources given the problems it encounters.
Spending calories keeping women strong and powerful who are NOT defending the family but instead taking care of it is taking calories away from the men who are defending it. If you have a woman defending, she can't be taking care of children.
Indeed, just the ability to create breast milk is a HUGE caloric demand, and that's completely ignoring gestating a child. Why would you have even more calories spent on size and speed if you're not going to use them?
There are other organisms where the female is larger than the male, but in most of them, the male is not used as defense. In some, the male is simply a means of moving sperm around:
http://en.wikipedia.org/wiki/Ceratiidae
No, this is not a comment on masculinity (or femininity). Humans are not Angler fish.
It *IS* a comment on evolution: It will, over eons, maximize the efficiency of its creatures. Any creature that is more efficient (all other things being equal) has an advantage over its competition. Hominids that spent their calories making those that defended the family bigger and stronger while spending the calories on those who gestated on gestation and breast milk ... rather than trying to make them equal in all things ... those hominids simply survived more often to the next generation.
We don't live on the plains of Africa any more and there is absolutely no doubt that that will effect our future evolution.