That's not the same as a limit on brain size in adults. (Nor even directly a limit on brain size in newborns: up to a point they could always get longer heads instead of wider ones.)

> (And even if those biological limits coincidentally lined up with general limits on intelligence, just the fact that AGIs will be able to duplicate themselves or share knowledge directly would itself be a huge practical increase to their intelligence over us.)

Yes. A human with access to a calculator is much more capable than one without. An otherwise human-level AGI with hard-wired direct-'brain'-access would be even more capable---without having to manipulate the calculator with clumsy fingers and interpreting its outputs with a general visual system.

Add direct-'brain'-access to Google and Wikipedia and to huge amounts of raw storage, and your human-level AGI would already be super human.