A very interesting question.!!
Some time ago I read an article dealing with the synchronicity of the
motion of fluids, (I believe it was in The Scientific American) Part of
the article dealt with the insignificant deviation in blood pressure
between an elephant and a mouse. Since both a very large man, and a very
small woman (providing both are healthy) will have the same blood
pressure, it may be that this is the reason the ratio of brain size to
body weight is more important than just size alone. I.e.the larger the
brain,(in proportion to body mass) the more area for the same amount of
blood to feed.