Our head doesn't have to be bigger to have more of organic brain. Our head doesn't have to be bigger to have more brain. People with Hydrocephalus have reduced brain volume due to fluid accumulation in their head, yet some of them are still able to function relatively normal with the condition and some don't even know they have it. It seems human's brain can still function even with reduced volume but more dense and this is not that unique at all, some small animal make up for their small head by having a denser brain neuron eg. Goldcrest has almost 50 times neuron density per mg compare to a human brain. Of course I don't know how all this would translate to mental capabilities but it still shows that we can have the same brain mass even with a smaller head, and it would be interesting to see higher neuron density in an average size human head as well.

Maybe one day when AI is smarter than humans but is still willing to cooperate, people might relies on AI to think for them for every little tasks. Wouldn't that make advanced AI kind of a mental aid too?