Two possibilities I can think of.

There is a lot of airspace in the wad, but the force required to collapse the legs in concert with the lower pressures involved, keep pressures from spiking.

The other is that the shot charge is easier to force down the bore so pressures don't get a chance to spike. When a 500 grain bullet hits the rifling, it's going to want to stop, spiking pressures.

Tim