weapon ranges

I have a question for everyone.  Has anyone sort of wondered why the
weapon ranges are so weird.  I mean a light catapult can shoot three times
as far as a heavy catapult, now the only way I could see this happening is
if the weight of the shot were increased more than the power of the
machine.  However, in the real world dark ages trebuchets drastically out
distanced the smaller catapults.  The same goes for ballistae.  What do
you guys think?

				Rob