There are certainly many ways a more intelligent species can go extinct despite, rather than because, of its intelligence. Our noble species could even be he culprit. A Georgia trailer park is sufficient evidence to the human breeding capacity. Or it could have been a virus, climate pressure, hell, space aliens are even a better explanation than they self reflected themselves to death (Richard, is it really THAT easy to get a PhD in evolutionary biology?).

But brain size and intelligence are only loosely correlated. Perhaps their brains were larger because they were less efficient. "Brainiac" was thousands of times larger than my iPod. Why did we stop using such a powerful computer? Was it just too much AI? Did we have to destroy Frank Herbert's thinking machines? Or was it just a simple "size doesn't matter" circumstance? Just maybe, the larger brain burned too many calories per thought to compete with a slightly improved homo- design using more proton pumps in nervous cells, or better transmitters and/or hormones, or tweaked receptor sites... Without tissue, it is very difficult to get solid answers.

When it comes to anthropology and paleontology, a guy I once borrowed money from gave the best advice (in a dark cornfield): Keep digging.