The thing is it's not just size, it's personality. St Bernards, Newfoundlands, and Great Danes are absolutely huge - much bigger than XL bullies or other pit bull types. Yet they've never killed anyone because it's not in their genetic personality traits. Not only are fighting breeds strong and athletic, they're naturally violent, and exceptionally tenacious which makes them desire to finish the job (ie, fight with the aim of killing).
In fact I know a St Bernard x Newfoundland with a useless owner who hasn't trained her at all. The dog is a nuisance, but completely and utterly harmless. Most large breeds don't need to be muzzled, also pit bulls and American Bullies smaller than XLs are medium breeds, yet they're still capable of killing adult men. It's only fighting breeds, and aggressive or high prey drive breeds like huskies, rottweilers, Belgian Malinois etc that need to be muzzled.
I agree that owners should be charged for whatever their dogs do. If their dog kills someone, they should get manslaughter, or murder if they deliberately set their dog on them. If their dog maims someone, they should get GBH, or GBH with intent if it was deliberate. The legal liability would make at least some owners more careful.
I suspect that a lot of XLs are being dumped recently because the owners know they are aggressive liabilities and worry that registering their dogs will basically be a paper trail that makes them legally fully liable if they attack anyone. But they feel too mean to put them down, or take them to a shelter with the same outcome, so they just dump them and leave it to fate.