Size does matter, but sometimes its what's on the inside that really matters. Who would guess this situation could happen; not the big guy, that's for certain! Life is like that, full of surprises! Consider the following Bible passage and realize that if you are following God faithfully, you are not alone:
2Ki 6:15 And when the servant of the man of God had risen early, and gone forth, behold, a host compassed the city both with horses and chariots. And his servant said to him, Alas, my master! how shall we do?
2Ki 6:16 And he answered, Fear not: for they that are with us are more than they that are with them.
2Ki 6:17 And Elisha prayed, and said, LORD, I pray thee, open his eyes, that he may see. And the LORD opened the eyes of the young man; and he saw: and behold, the mountain was full of horses and chariots of fire around Elisha.
2Ki 6:18 And when they came down to him, Elisha prayed to the LORD, and said, Smite this people, I pray thee, with blindness. And he smote them with blindness according to the word of Elisha.
I guess what explains the picture the best is that there is something inside the small dog that gives him the strength to stand up to the big one. In other words, it's not the size of the dog in the fight, but the size of the fight in the dog.