I don't think its down to the individual dog as a whole. I think some breeds are just naturally passive and some more natually dominant. Like my japanese chins, compared to my staff.

My staf can do far more damage with her larger mouth and teeth and shes just pure muscle. Staffs tend to be rather boisterous(sp) by nature anyways, shes vocal and vigilant. My JCs are extreamly laid back smaller dogs, simply pref to be waited on hand and foot, they don't even bark, have no oral fixation(dont chew or bite anything)etc.

Not that she ever has, and most likely never will, but it would be my staff who would ever do actual harm to someone.