Astoundingly, batikwong here is actually citing a bit of evolutionary theory correctly. It's called allometry and basically states that various phenotypic metrics will tend to increase in proportion with each other, usually because of a common genotypic control mechanism.
So as an species increases in average size, it tends to do so in proportion (barring selective pressure on any particular characteristic). Hence pygmy hippos are smaller versions of hippopotami, etc.
The classic example, demonstrated in a paper by Stephen J. Gould (Nature 244:375-376, 1973), are the enormous antlers of the Irish Elk. By plotting the antler and body sizes of individuals from many different elk species, Gould was able to show there was a strong power relationship between antler-size and body-size, and that the Irish Elk fitted exactly to that trend. The enormous antlers of the Elk were not the direct product of selection, they were simply the consequence of that elk species having grown larger over time.
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So why, when our ape ancestors got smaller, did our heads and brains not get smaller too?
Two reasons.
1) Our ancestors were smaller than we are, so we have actually increased in size as a species.
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2) There's that pesky phrase "barring selective pressure on any particular characteristic". If some change in body-size is generally beneficial, but some allometric change is disadvantageous, then when the cost of one balances the benefit of the other, the trend to change body size should cease. Unless there was some other variation that changed the allometric relationship between the two characteristics. If this were so, then there is not only a selective pressure for the change of body size, but for the weaker coupling as well. Hence while the brains of early homonids might have initially decreased in size with a decreasing body (had this actually happened), if this was as disadvantageous as seems likely, the trend would quickly have changed as those individuals whose brains decreased less gained ground on the rest.
Not a completely stupid question, then, however stupid his conclusion.
