the lack of a 'recoil shoulder' at the top of a single-action allows the revolver to roll backwards so far in the grip that the thumb can then readily be brought into play to cock the hammer for the next shot. the lightweight structure of many of the older single-actions also was helped, i believe, by the action of the revolver rolling in the grip of the shooter.
the quick-fire capability of the double-action calls for a recoil shoulder so that this 'heavier-design' of a revolver can assist with getting the firearm back on target. sure, we want a potentially greater volume of fire when we work with a double action, but we also want it to come back on target to make that volume useful. i think that's part of the design parameter, since the weight of a double action helps absorb the recoil.
at least that's how i see it,
ss'