Why doesn't brain size matter? Why is a rat with its tiny brain smarter than a cow? Why does the cow bother devoting all those resources to expensive gray matter? Eliezer posted this question in the February Open Topic, but no one took a shot at it.

FTA: "In the real world of computers, bigger tends to mean smarter. But this is not the case for animals: bigger brains are not generally smarter."

This statement seems ripe for semantic disambiguation. Cows can "afford" a larger brain than rats can, and although "large cow brain < small rat brain", it seems highly likely that "large cow brain > small cow brain". The fact that a large cow brain is wildly inefficient compared to a more optimized smaller brain is irrelevant to natural selection, a process that "search[es] the immediate neighborhood of its present point in the solution space, over and over and over." It's not as if cow evolution is an intelligent being that can go take a peek at rat evolution and copy its processes.

Still, why don't we see such apparent resource-wasting in other organs? My guess is that the brain is special, in that

1) As with other organs, it seems plausible that the easiest/fastest "immediate neighbor" adaptation to selective pressure on a large animal to acquire more intelligence is simply to grow a larger brain.

2) But in contrast with other organs, if a larger brain is very expensive (hard for the rat to fit into tight places, scampers slower, requires much more food), there are other ways to dramatically improve brain performance - albeit ones that natural selection may be slower to hit upon. Why slower? Presumably because they are more complex, less suited to an "immediate neighbor" search, more suited to an intelligent search or re-design. (The evolution process would be even slower in large animals with longer life cycles.)

I bolded "dramatically" because the possibility of substantial intelligence gains by code optimization alone (without adding parallel processors, for instance) also seems to be a key factor in the AI "FOOM" argument. Maybe that's a clue.