When we pass a small furiously barking dog or two, my dog knows that he is to ignore the dogs and walk by my side. But every once in a while, it gets to him and he will give a short silent mock sideways lunge. The small dog usually reacts with an even louder bark and backs away, revealing his fear. Then my dog appears satisfied with himself and walks on. He knows that I disapprove of this behavior, but I do have a little sympathy for him. It's hard being barked at like that all the way down the street and having to "take it".
I've seen him do the same thing with dogs blustering at him from behind fences. He will "take it" usually, but every once in a while he will rush the fence as if to say, "Oh yea? Let's see what you got!!" The barking dogs usually back away or run, revealing that it was all a 'show'.
Anyway there is weirdly complex fascinating social communication between dogs!
I do believe your pup could have been proud of himself after 'defending' your smaller dog.
Rumo ~ rescue shepherd/husky mix