I don't know. Do dogs take their size into account when their instincts tell them to act? In other words, 'No way Jose, I'm too small'?
I have, as we have all heard if not seen small dogs take on large dogs, wolves, bears, and so on. The dogs do not always come out n top but they try. Defending someone?
We rescues two "speed dogs" or greyhounds some years ago. They both seemed to think of themselves as small dogs! Curling up on our sofa left no room for us!