Yes, large brains are obviously very important. The article says the maximum difference is 100 cm³ while the largest brains are about 1500 cm³, so on average 7

Yes, large brains are obviously very important. The article says the maximum difference is 100 cm³ while the largest brains are about 1500 cm³, so on average 7% larger means 2% smarter. But I think a couple of things come into play - one is that our genes might be optimised for a certain size and it's not simple to scale, but requires reprogramming. At least in some species, larger brains just means larger neurons, not more of them.