Actually you are on to something there. About 8 pounds that a woman gains while pregnant are there as a nursing reserve. If a woman does not nurse her baby for at least a month or two, she will have a much harder time loosing this weight. If females were not very mobile for a few days after giving birth, something the evolution of a larger head might have influenced, then having this reserve to live off of and produce milk from had a distinct survival advantage.
It does a body good.