This is a Fish Crow V Immature Great Blue Heron.
I photographed this at rookery in Tampa Bay.
From my experience, it's not the size of the bird in the fight, it's the size of the fight in the bird.
Just don't mess with Mama. I have seen tiny birds attack very large birds protecting their nest.
This behavior would be completely out of character if not for protecting a nest.
I have also noticed that birds are more aggressive protecting their babies than they are their eggs.
Am I full of it or are these creditable observations?
PS Welcome John!