The difference between the two pistons are quite apparent. The "LIGHT" piston has a metal ring that keeps it secured in the gas block. This makes it relatively easy for the gases of lighter loads to build up pressure in the gas block, sending the piston rearward.
The "HEAVY" gas piston has several grooves cut in it that seem to be designed to build up the gases prior to moving rearward. This is why this piston does not stay in the gas block like the "LIGHT" piston does. When you are firing heavy loads with alot of gas pressure, you need to have somewhere for that gas to go that will not cause unnecessary pressure and wear. This "HEAVY" piston makes it to where the extra gas pressure has to fill in the gaps between the piston and the gas block before the piston can drive the bolt carrier rearward with enough inertia to cycle the action.