Ahahahaha, facepalm, I had the Macavity song running through my head and completely skipped that it came from the poem. Derp.

Agreed about the big head = big brain...Doyle was all over that, I think. It's waved about as a smug deduction in the Blue Carbuncle, when he's telling Watson that the owner of the big hat must obviously be a scholarly man. Okay, buddy, if you say so!