I think you guys missed the point.
Eating meat didn't magically cause larger brains. Eating meat
allowed for larger brains. Also, "larger" is the wrong thing to focus on. "More metabolically active" is correct in terms of Kleiber's law.
To reduce the argument to a strict syllogism:
1) Kleiber's Law establishes that animals of a given total size will have some maximum amount of metabolic activity.
2) If, over evolutionary time, one organ decreases its metabolic requirements, the others must pick up the deficit. If one organ increases its metabolic requirements, the others must decrease theirs.
3) Therefore, for humans to evolve more metabolically active brains, they must have decreased their metabolic expenditure elsewhere.
So, left with that conclusion, the next question that is "what did we give up?" Again, a syllogism:
1) The above syllogism establishes that for humans to increase the metabolic expenditure of their brains, other organs must have given up some metabolic activity.
2) The organs that appear to have given way are those related to digestion. This is evidenced by the fossil record and contemporary comparisons.
3) Therefore, for the human brain to increase its metabolic expenditure, a change in diet must have taken place.
Finally, we ask ourselves what change in diet:
1) The previous syllogism establishes that a change in diet must have taken place for the human brain to expand.
2) The most widely available food which was calorically and nutritionally dense enough to enable humans to live with smaller guts is meat.
3) Therefore, for the human brain to increase its metabolic expenditure, humans must have started eating meat in preference to other foods.
Notice the causal links here. Our brains were expanding, so we had to eat meat. Not, we ate meat so our brains magically grew.
The selective pressures that lead to humans evolving active brains are not considered by this theory. Those selective pressures demanded humans develop more active brains. As those pressures selected for those brains, humans that ate meat were able to support them, by giving up their large digestive tracts. The more meat you ate, the less you need to use your guts, the more you could use your brain, and the more likely you were to survive whatever selective pressure existed that was pushing you towards using your brain more. Thus selective pressure for meat-eating due to its ability to support active brains.
That pressure or pressures could have been anything. The claim is that eating meat enabled or was
necessary for us to evolve large brains, not that it was the
sole factor in our development of large brains.
EDIT:
Here's an important quote from the article:
Any or all of these hypotheses may be valid, but the problem isn’t really as much a matter of why as it is a matter of how. Other primates deal with groups and have complex foraging strategies; and many deal with social problems within their groups, and some even hunt. Yet they still have small brains. (Granted, their brains are larger for their size than those of other mammals, but primates sport small brains as compared to humans.) How did the human brain grow?
Note the emphasis on
how as opposed to
why.