Yes many dogs over 20kg are capable. Most breeds do not have the temperament to do this, but there has even been instances of medium sized dogs like border collies killing people, and huskies have killed lots of children and babies, so yes, if the dog is determined enough it can happen. Even labs and golden retrievers have maimed and killed people so even the "safer" breeds can flip and "go bad" (its rare but its possible).
If 20kg is too small, then up the limit to 25kg or 30kg. But basically dogs over a certain size have the weight and bite force to do enough damage to kill. Pit bulls are on average 25kg yet they can do immense damage, and staffordshire bull terriers are only around 15 -20kg and they can also severely maul people (I've seen it myself).
The point is you need to draw a line somewhere. Where will that line be? As has already been stated if you ban breed 1, people who use the dogs as status symbols or protection just move onto breed 2. It's a never ending cycle until you've gone through all the large potentially dangerous breeds 1 by 1.
Rather than wasting time doing this, ban those people from owning any large dogs at all (put the weight limit wherever its deemed to prevent the most maulings), and that is more effective since they can't just move onto another potentially dangerous breed and stay within the law.
I guarantee you that this XL bully ban will not prevent more deaths or maulings in the future. It will just be a different breed causing it.