I agree. But it's "easier" to evolve a slightly larger brain with the same architecture, than to discover a new, more efficient architecture.
In general we should expect a larger brain (ours consumes 1/4 of our total energy) to pay for itself in more actual intelligence; if it doesn't (the architecture can't easily scale) then the smallest (still working) brains will win.
For each species of bird, either
1) general scalable intelligence isn't supported by their brain's architecture (so you can't just grow more processing power)
2) birds lack the physical attributes to profit from any more general intelligence than they already have
or
3) it (general intelligence) just hasn't evolved yet
In general we should expect a larger brain (ours consumes 1/4 of our total energy) to pay for itself in more actual intelligence.
I don't think we should generalize from the single data point of recent human evolution. Are we sure larger brains tend to give greater general intelligence in other lineages? Has this been checked? Is the intra-species brain size variation large enough to check this in species where we already run intelligence tests?
The fact that human brains recently became unusually large can be explained by other theories. For example, it ...
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.