Brain size is certainly an issue but from my understanding it's more about the constraints of childbirth than metabolic cost that are restraining it. When you'

Brain size is certainly an issue but from my understanding it's more about the constraints of childbirth than metabolic cost that are restraining it. When you're resting flat on your back your brain uses up 10 of the 100 watts you're dissipating but the variation in brain size given other constraints is pretty low and I'd expect plausible power differences from that to be pretty small too. And brain size only has something like a .3 correlation with IQ so other factors like efficient myelination or organization are probably much more important. I'd expect better myelination to reduce energy consumption if anything. As to organization I'm too unfamiliar with the scaling laws of neurons to be sure but I'd suspect brain organization that increased intelligence at least wouldn't increase energy usage much. So I'm still inclined to view intelligence as being constrained, evolutionarily, by issues of complexity rather than tradeoffs between efficiency and effectiveness as with height.