Sorry i joined the party late but I watched a relevant documentary a few years ago. From what I remember, the brains of Homo Erectus (the predecessors of homo sapiens) did increase in CCs after adding fish to their diet and then either Homo Erectus or Homo Sapiens added meat to their diet which again increased the size of their brains. However, I think veganism is really cool because I see it as possible largely because of our technological evolution that has occurred through innovation and ironically brain capacity. I guess this same innovation led us to domesticate animals too and is what will eventually lead us to create synthetic meats or meats and dairy made of stems cells or algae.

Getting back to the point, brain size does not correlate with the greater ability to adapt or intelligence. Neanderthals had larger heads and brains but died out (while homo sapiens lived on and grew) because of their ability to adapt. Neanderthals were also built larger allowing them to use large spears to kill their prey and have better eye sight suited for the foggy european hills they were accustomed to hunting. But humans were smaller framed (and therefore were able to run faster and farther) and used smaller and lighter spears allowing them to throw their weapons from far distances. However, one of the greatest reasons why homo sapiens beat out neanderthals is because of the size of our communities. In the first hundred years of our existence, humans lived in communities of 50-100 people while neanderthals lived in communities of about 12. The idea of community and the ability for the community to persist, the divying up of duties, etc allowed us to proliferate and eventually push the neanderthals towards the southern tip of italy where the bones of the last neanderthal was found. Unfortunately, our disjointed communities and the large communities that gave us an advantage may be what spells the end of homo sapiens (and all life) in the future. Anyways, sorry i got into a tangent on homo sapiens and neanderthals but i think looking at the earliest human species provides a great understanding of evolution and brain capacity and how they relate.