Because, like true terrier temperament, they won’t let it go. They’re tenacious, holding on and determined not to merely say “get out of here”, but to rip apart.
I have a 4 yr old female rat terrier, Amy @ 15 lbs, and had been keeping my son’s 1.5 yr old 90 lb shepherd (McClane). When McClane got her mad, she would just bite him in the face. McClane is a big puppy and so in love with my son that when I dog-sit he moans in grief at being separated. He sometimes accidentally hurt Amy with his huge feet, but never, ever hurt her intentionally. She did.
Small dogs have it rough. Imagine living among people who are 20 feet tall and weigh 800 pounds when you are 5 feet tall and weigh 140. I’d be shaking and wanting to run while screaming, too.