I wonder if they might have been able to make a helmet that DID fit over a head without being too bobble-heady if they hadn't insisted on incorporating the opening face plate. Maybe not. To my mind, the biggest problem isn't actually the size of the helmet per se, but rather the fact that the helmet has a face on it that is WAAAY bigger than a usual minifigure face. So basically, instead of just looking like a big helmet, like so many other LEGO helmets do, it looks like a big HEAD, which is freaky. I actually think it looks pretty good with the faceplate OPEN, so I'm not seeing that huge face.
All IMHO, of course.