If you take a look at how a 2h and a polearm are being held you see that the 2h weapon is held at the hilt and the polearm somewhat in the middle.
So the 2h stab gets all its length + the arms of the wielder, the polearmer gets... well dunno, pretty much nothing in comparison.
There is a list of bonus and malus to length with the different attack direction, but dont know where.