big traps probably have to do with how wide your shoulders are and how efficient the leverage system is in the whole traps/shoulders complex. I have naturally big traps, so much that when I turn my neck to the side it almost looks like I have a hump back or something, and I rarely do shrugs or anything, my traps have to be bigger because I think they are simply less efficient... I need more muscle to move the same weight since the shoulder is farther away from the neck. it does look powerful, and it looks great if u have a small waste to go along with it, but it's deceiving, im not actually all that powerful
if u look at most successful power lifters, they dont have wide shoulders, they have tight frames, when u have sloping wide shoulders, your shoulders and traps have to do more work to accomplish the same thing