The relationship of brain and intelligence is very complicated and is under research.Few studies have shown that there may be a relationship between larger cortical size and higher iq.
Nerve branching plays an important role obviously.Because it may help in interconnecting various anatomical structures of cerebral cortex,which is essential for maintaining complicated functions of brain like -Intelligence

For further details refer to
/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3181994/