Its not the size of the dog
When you think about it, the statement is not what people under a certain height or weight want it to say, that the little dog always has the bigger heart and that you should never underestimate anybody. The statement is simply that the size of the dog doesn't determine the dog's heart. There are short bullies who think a larger opponent is short on heart, and they're really everything they say they are against.
Its not the size of the dog in the fight is a way for some folks to get support, but the reality is heart depends on both the underdog and the favorite, sometimes the underdog is a pussy who's already thrown the fight, and sometimes the favorite is not a favorite because of hype, it depends on the dog.