FWIW I think the thingg is that if you get bitten by a little fluffy dog (AKA a rat) chances are you will have some nasty scars but you will live to tell the tale but if you get bitten by a big mastif kind of dog you just might get killed.
That's one of the reasons that I got a small dog. I think that any dog will bite if it is pushed too far so I may as well have one that won't do to much harm if it is pushed beyond tolerance.
Then again bull terriers and bull dogs have high pain thresholds so their tolerance may be greater.