Bullshit.
Most of the animal kingdom defers to aggressiveness, not strictly size.
This is why honey badgers and wolverines have such fearsome reputations. They'll scare off a full sized grizzly bear.
Even amongst dogs, put an unhinged pitbull up against a saint bernard twice its size and the saint bernard ain't gonna want none of that.
I've even seen with my own eyes a back alley tomcat go for the eyes and face of a dog who was an actual pit fighting dog and that dog took off running.
Obligatory picture and some pretty awesome videos:
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(a cat inflicted this per the article i stole it from)