Raceyb, I like the simple way you expressed that.
I freely admit, I'm a rookie, here to learn... which is why I dont post much, so I'm hesitant to disagree, but...
My observations are that with my electric smoker, once I have it up to operating temp the amount of meat doesnt have a big effect on cooking time. Some effect, yes, but not much.
As you say, more meat will require more energy/heat to cook, and should therefore take longer. But here's the thing: I also have observed that when I have more meat in the smoker, the element is "on" a lot more that when I only have 1 small butt in it. Which means it is putting more energy in during the same time period. Charcoal, wood, propane - the heat output is more constant, charcoal's heat output doesnt go up as you add more heat-absorbing meat. With the electric, I think it does.
Another observation: If I add side dishes - like maybe Dutch's Beans and a half dozen spuds - in the middle of a smoke, that does not change the amount of time it takes the meat to finish cooking.