I think what your seeing are on treadle hammers as aposed to helve hammers. And in reality you dont need a heavy truck spring on a helve.
The spring is there because somthing has to give. If the mechanism is built to just kis the anvil all is good untile you inteoduce a peice of steel, and i there isnt a spring to provide some give, somthing breaks. You should see what happens to a punch press when that hapens, thing small bomb, compleat with shrapnel. So the spring gives when the hammer hits the workpeice, so the drive traincan make its compleat cycle. Not realy nesisary when your leg provides the power.
On a note, two light of a spring and you can reach a point of resinence, and the hammer will flote, wile two heavy a spring and the hammer will stall, and the weekest link in the mechanism will fail.