Since my DS5's head has always measured 98%ile, I'll chime in...
The studies attempting to correlate IQ with head size have been mixed. Most of the data is old and suffers from bias of some kind. There was a study in 2003 correlating rapid gains in head circumference during the first year of life (i.e. going from 25-50%ile to 80%ile plus)with development of autism.
Interestingly, Einstein's brain weighed no more than an average man his size. This may be because he died at a rather advanced age (in his 70's I believe), and we know that there is a normal amount of shrinkage of the brain with age. BUT - Einstein had two notable differences - his biparietal diameter (measurement of the brain from one parietal lobe to the other) was significantly larger than average. The parietal lobe typically contains "association areas" where the brain makes sense and analyzes primary sensory inputs (vision, hearing, etc). So, perhaps Einstein was able to make more "associations" than the average person. Also, his brain had significantly more folds in it, which, phylogenetically is correlated with intelligence - humans have more folds in their brains than, say, great apes.
