The largest brain/body ratio in vertebrates is found in birds.It's not the size of the boat but the motion of the ocean.
But my point was that sight is one of our most brain demanding senses. When you add in color and sharpness it eats up a great deal of wet wear.
That is why birds have such a large brain to body ratio.
If Neanderthal had better vision then Cro Magnon it might explain the larger brain capacity.
Vision is one of our most complex and least understood systems. We take in a whole lot more then we actually see and process. Also possible that they could process more.