Your question reminds me of an extremely rare medical case that was discovered in Marseille, France.
A young man, finishing his PhD in Mathematics, checked in at the Timone hospital (if I remember correctly) because of constant headaches.
When they performed a MRI, they realized that his skull was full of liquid, and probably for a long time, since his brain had shifted to being a layer of grey cells of around 8mm thick on the inside surface of his skull. The cause was a childhood-disease, when he had liquid forming up in his skull. They placed a drain inside to derive the liquid out of the brain, but it must have gone blocked and the liquids pressured his brain to the boundaries of his skull.
I am afraid I do not have access to Google to find a link to this case, but to the best of my knowledge, 3 cases have been identified in Marseille. It is, to me, one of the most clear example of how the brain’s size and structure probably has little to do with « intelligence » as we know it!
On a side note, I know that another lead followed by some scientists is related to the « astroglie » (please forgive me, i only know the french term

) which forms a parallel network with neurons in the brain.
And regarding intelligence and IQ, I find IQ tests to be such a pale indicator of one’s abilities. They don’t test Emotional intelligence, or language-learning abilities, or artistic abilities, which are also a very important part of our intelligence. I remember a classmate at school when I was 9, who was always refered to as « the special kid » by teachers because he had huge trouble reading, writing and assimilating lessons. And yet, when we played in the woods, he displayed tremendous ability to navigate the forest, never got lost, and had a near-photographic memory of places he had visited. He is now a very gifted forest manager, and to hell with tests that labeled him a « special needs child ».
A small quote I like on the subject:
Albert Einstein wrote, “Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.” The question I have for you at this point of our journey together is, “What is your genius?”
So much yet to be discovered, what a fascinating time it is for humankind