Oddly enough I was thinking about this the other day, my dog was rehomed because he is a biter. He weighs 15kg, when I first got him (before the hard work began) if he was showing signs of aggression I could physically remove him from the situation, even if he didn't want to go. I could hold his jaw shut and pick him up if needs be. I know of many small dogs who have bitten but because they are small, they have smaller mouths, smaller teeth, can't knock you over (might trip you up) because they

Don't have the body weight, because of the lack of height they are more likely to give a painful puncture below the knee. I don't believe that a bigger dog is any more or less likely to bite than a smaller one but I do think that they have ability to do more harm just because of their size

Actually not true. A Jack Russell attacked a child and killed it some years ago in a family home in the U.K.