When you imported the head into Poser did you remember to uncheck the Percent of Standard Figure Size box?  If you leave it checked and set to 100% you are telling Poser that you indeed want the head to be the same size as your figure. So Poser is just doing what you are telling Poser to do.  Where-as if you uncheck it, it imports the head in at the size your modeling program exported it without changing its size.  Also if you uncheck all the other boxes it keeps the head at the same location as it started at, assuming you didn't move it in your modeling program.