I knew nothing about Einsteins brain before going to this page, but hypothesizing based on Tsadi's facts I have come up with this, which might help
White matter's purpose is to carry neurons from areas of gray matter to other areas of gray matter. With the excess of white matter, there was probably also increased neuron usage. This would probably cause him to make connections between certain things more quickly, which would definitely aid Einstein with developing ideas.
Einstein's inferior parietal lobe was large, and that dealt with interpreting sensory information from the rest of the body. It has been thought that this also dealt with separating visual inputs from the subject's perception. This is somewhat of a stretch, but one might be able to say that Einstein literally did not see things the way everyone else did.
and as for
there is actually a possibility that 'nurture' did alter his brain, Macrophaly has been known to develop over time due to environmental conditions.
Again, no hard scientific evidence that i could pull up right now, just me surmising. Although i suppose i could go a-poking around to find the articles and such that i read that informed these ideas. Also, I am just a high school student so if any of this doesn't make sense, I apologize, my education hasn't really gone in depth with ANYTHING as of yet.