The definition of an abnormally large or small head is based on statistics and is arbitrary.  A normal child can have a head circumference which is borderline in either direction.  The size of the infant's or child's head is determined by the growth of the cranial contents and the structure of the cranial bones.  The rate of growth is influenced by heredity, general health, function of other systems (cardiac and endocrine) and the child's age, weight and race. 

A measurement that is statistically abnormal should be supplemented by observation and measurement, if possible, of parents and siblings.  After 2 years of age the head circumference should be smaller than the chest circumference.  It has been documented that we Guatemalans, have a smaller head circumference than the one documented on the NCHS tables, the important data to consider is that the head should grow parallel to this tables as the rate of growth is really the important information to consider.