I am unaware of any proven direct correlation between physical brain size and IQ, at least in individuals.
Might be different in large groups such as ethnies or “races.”
There is most certainly a positive correlation between brain size and intelligence.
Correlation is not causation, but good health and nutrition during growth is going to increase both.
It is thought that men score better on IQ tests because their brains are larger on average. When you compare men and women of equivalent brain sizes, women score better on IQ tests.