Here’s an excellent paper on head size and IQ by Catharine R. Gale, Finbar J. O’Callaghan, Keith M. Godfrey, Catherine M. Law and Christopher N. Martyn.  It seems that the correlation between head size and IQ is caused by brain growth that occurs during infancy and early childhood.  Brain size at birth is relatively unimportant.  For example, in a huge study of 248,000 young men (Lundgren et al., 2001), a 4 standard deviation gap in birth head size was associated with only a 0.03 standard deviation gap in adult IQ, which, according to Pumpkin Person math, suggests a correlation of only 0.0075  (0.03 SD/4 SD ).

However in the first nine months of life, the correlation between later IQ and current head size rises precipitously, and remains, even when controlling for many other variables such as mother’s IQ , education and breast feeding.  Brain growth from nine months to nine years is also crucial.  The authors write:

There was some evidence that the two periods of head growth differed in importance as regards verbal and performance IQ. Verbal IQ was associated with head growth between 9 months and 9 years of age, while performance IQ was more strongly associated with head growth in the first 9 months of life

The authors also note:

There was also evidence to link social class and head growth. Between birth and 9 months of age, head growth tended to be poorer in children whose mothers came from social classes III, IV or V than in those whose mothers came from social classes I or II, although this trend was not statistically significant. But during the period from 9 months to 9 years of age, there was a significant difference in head growth between the classes. Children whose mothers came from social classes III, IV or V experienced markedly poorer head growth than children whose mothers came from social classes I or II.