[QuickSilver]

Oh, I didn't read that quite right originally. You are saying he only has aggression with big dog PUPPIES. That isn't that surprising, actually. There could be a number of things there.

First, it's VERY common for adult males to bully other teenaged males. I understand you may not have had time to check for this, but does he only pick on males? Is Couver neutered? That would explain out and out aggressive behavior.

Even if it's not "bullying", an adult dog may take it upon themselves to teach whippersnappers their manners. Generally puppies accept this, so I don't think that's as dangerous as if he were challenging large breed adults.

Aside from that, puppies move differently than adults, just like human kids. Perhaps their movement just irks him, or appears threatening.