Actually, I didn't say brain size was dependent on intellectual ability. I said the contrary: Intellectual ability was dependent on brain size. If we said brain size depended on intellectual ability (or rather the need or lack of need for it) then we have to also depend on food intake for this brain to grow in the case we need a greater intellect. In other words, the theory works only one way while if brain size depended exclusively on food intake then it works both ways.

If we still say brain size depends on intellectual need, then it stops working because even though we don't need to be as smart, we still have ample food intake to sustain a big brain as the example of a dense population. But the evidence regarding dense populations seems to refute this idea outright for the simple reason that dense populations seem to be sicker than sparser ones. If we're less healthy than sparser populations, then obviously our food intake is to blame. If our food intake is to blame for our health, I doubt it would allow our brain to stay big. Ergo, it's not the fact that our population is dense, rather it's the fact that our food intake in dense populations is inadequate, i.e. we are malnourished. It always comes back to food intake as the primary determinant of brain size.

If we accept this theory:
http://www.proteinpower.com/drmike/...arians-part-ii/

Then we must accept that the shrinkage was due to a lack of adequate nutrition.