This is an interesting point.
When I think about it, I only perform a maximum of 9 working sets for legs in 1 session(ex; 3 set Squat,3 set Leg Press, 3 set Leg Curl) , yet I also do 6 sets for Biceps…a significantly smaller muscle group(ex; 3 sets Barbell Curl, 3 set Preacher Curl).
Although it is only 3 less sets, the overwhelming amounts of fatigue and neural destruction imposed by those 9 sets of legs were far greater. I would go on to say, that you can’t necessarily say, a “bigger” muscle requires more volume…also this DOES tend to generally be implemented widely throughout the training scene.
I, for one, am absolutely positive if I tried to increase the # of working sets on Legs, to say…15 for example, that I would probably not reap any greater benefit , and would most likely induce myself into a coma for the next week lol.
Although it’s not the best example, watch Ronnie Coleman’s leg training.In one leg workout, he typically does ~2 working sets on Back Squat, then ~2 working sets on Leg Press, followed by some lunges or something that isn’t nearly as taxing for maybe ~2-3 working sets.
That’s really only about 6-7 working sets for legs…
Yet, you will watch him do the same,or even more volume(# of working sets) for a small muscle group like triceps.
It may seem strange, but if you really put things into perspective, it kind of makes sense.
Regardless - my own preference is about ~6 working sets for delts/arms/calves …~9 working sets for chest/back/legs.