Well,

We have human beings who seem to function normally in situation where their brains have actually shrunk to a fraction of their skull size. And we animals that can do things robots can't do but will very small brains indeed. So something like an intelligent algorithm seems possible with "small" "hardware".

Oppositely, it seems like hypothetically one could pretty easily specify an "intelligent algorithm" if one had a computer of limitless speed and memory.

So approaching AGI would seem to involve producing things of sufficient efficiency and there's reasons to think that's possible.

Also, think of whales with massive brains, with significantly less intelligence than humans. Also, check out how smart crows are, with a vastly different brain structure than mammals, and with only a fraction of our brain volume.