The small dog is the more dominant of the pair. Dogs don't factor size into things like we do. If the small dog is the alpha dog that is all the big dog understands.

This is common because an owner is more likely to be strict with large breed, which establishes it's place in the pack hierarchy. Owners tend to let a small dog's aggression (dominance) go unchecked because “it's just a small dog and can't hurt anybody”.

The result is what you see here. This is also why you may notice that smaller dogs tend to behave poorly (at least that's what I've noticed).

Of course, it all comes back to the owner. No bad dogs, just bad owners.