It's not so straightforward to judge threat levels. Seems to me that there are three related issues that interact with one another in not-so-straightforward ways.
1. The power of the dog, so unsurprisingly bigger dogs have stronger bites and have the potential to do more harm.
2. The typical temperaments of different breeds. Some breeds are more aggressive than others, some are more prone to mental problems than others.
3. The kinds of people that buy certain breeds. Clearly there is a self-selecting aspect here, which may skew the data. If a responsible, knowledgeable owner looks after an XL Bully and the dog is well socialised, how does that mitigate any risks from considerations 1. and 2.?