Of course it’s the breed. Those dogs that are bred for that sort of thing do that sort of thing. And a chihuahua, or various other breeds, couldn’t have inflicted that harm if it wanted to.
Not every one trains a pit bull for fighting. Or a Cane Corso or Dogo Argentino.
If you’ve seen “Pit Bulls & Parolees” on ANIMAL PLANET, you’d understand why they are not like the dogs reported on in the news media.