I read an interesting article a while ago about a Russian scientist Dmitry Belyayev who did an experiment to try to domesticate wild foxes through selective breeding (by selecting the most docile specimens for reproduction) and the foxs' heads (and presumably brain) shrunk as their became more domesticated (source: https://www.pelicanbooks.com/the-domesticated-brain/preface).

I have a theory that it's the same with people - It would be interesting to do studies. It's well documented that Neanderthals had larger heads/brains than Cro-Magnons.