Holding aggro

[Crass]

Am I understanding correctly that without procs or aggro-reducing effects like FD, tanks hold aggro by doing more damage than anyone else?

That always seemed odd because it forced casters to be "weaker" than melee classes both for damage and defense.

Anyways my question is: in high end fights, with all the tools like procs, how does the main tank's damage output compare to others'?