The Collaborative International Dictionary of English v.0.48:

macrocephalic \mac`ro*ce*phal"ic\, macrocephalous
\mac`ro*ceph"a*lous\, a. [Macro + Gr. kefalh` the head.]
   1. Of or pertaining to macrocephaly; having an unusually
      large head.
      [1913 Webster]

   2. (Bot.) Having the cotyledons of a dicotyledonous embryo
      confluent, and forming a large mass compared with the rest
      of the body. --Henslow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

macrocephalous
    adj 1: having an exceptionally large head and brain [syn:
           macrocephalic, macrocephalous]