infants' fontanele is still open and their bones are elastoc, so any increase of intracranial pressure will be compensated by enlargement of head circumference until one point where they can not compensate any longer. it is then when you may find papiledema.
In addition to the fontanelles, the sutural diasthesis that occurs following raised intracranial pressure help in minimizing the effect of the raised ICP.
it is still a good questions and the collegues provided the 'usual' explaination, perhaps also differerences in tissue dynamics (pressure/dilatation) may also display maturational differences ?