No, not really, I guess I will have to lay it out for you. It really boils down to this: The pitch circle of a fine thread is larger than a course thread. You can't talk about the depth of the thread because the deeper you go into the thread of the bolt, the further and narrower the thread of the nut gets. The point where the nut and the bolt have the same shear strength is at the pitch circle. So, again, it's the size of the pitch circle. Bigger diameter means more circumfrence means more material to shear.
Are we going to have to put some bolts in the vise again to prove my point?