Kristen Stewart is mean as a snake! I can imagine* her beating someone like Bella, who while bigger isn't really "nasty" enough to make a good fighter. (*unless I missed it, I haven't actually SEEN either of them in a fight in their films or on a youtube vid)
Now: 3" in height COULD be an advantage for Bella, but 3 pounds in weight is nothing! In fact, at about the same weight, with her shorter body, it could actually be that Stewart is the more muscular of the two.