the limiting factor on brain size is the size of the pelvis opening - for birth
the pelvis opening has got about as big as it can be - and still allow a woman to walk (!)

therefore the only way to get more brain power is to pack more into the available space

it might be that 'grey matter' (the more dense brain material) came about more recent (then the other type of brain matter)
over time the entire brain may well be made of 'grey matter'

It's not the size, it's the number of squiggles... or what ever they're called. It's the square footage, so to speak of the surface area affecting how many grey you have.

The more folds and valleys, the more grey cells, the more brain power.

Well, according to my "field" of knowledge, it is not size that really matter.
It is the number and the speed of "connectivity" or connection of the neurone.
In other word, it depends on the effectiveness of the communication of the neurone.