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Correlation within the current human species only, there was development other than just size from ancient history that you aren’t accounting for.

If you’re talking about the 10% myth, I wasn’t referring to that, but there are documented cases including at least one person missing most of their brain from a long-term condition where the brain filled with fluid, leaving a thin perimeter of brain matter, and they were otherwise still fully functional except for weakness in one leg. A total mystery, and provided proof that the brain can rewire itself and doesn’t need most of its volume to function correctly, and suggested evidence that the brain uses what’s needed, but that most of it isn’t required.

edit:

https://qz.com/722614/a-civil-servant-missing-most-of-his-brain-challenges-our-most-basic-theories-of-consciousness

Note that while it does say he was of below average intelligence but not mentally disabled, that’s not the point. The point in me providing this here is that losing most of his brain over time didn’t affect the intelligence he naturally had, or affect other brain function, so he was fully functioning and normal as far as anyone was concerned until he went to check what was going on with his leg. Providing proof that most of the brain probably isn’t needed, and possibly redundant to compensate for loss.