It depends how big the space is in your skull(cranium), if it gets too big for this the pressure would cause damage. I think about 2-2.5kg. The important thing with brains is the bigger the brain doesn’t necessarily mean it is smarter
I’m not sure what directly regulates brain size. Certainly the size of the skull would regulate this to some extent and i would imagine that there is quite a bit of variability is absolute size between individuals.