I can't say I'm an expert or that I'm recommending this method, but one reason is he could be viewing smaller dogs all as his subordinates and it will merely just take some consistent discipline to show that attacking smaller dogs is unacceptable. Getting the snip will probably calm him down, but I can't say that this is 100% the reason for this. If he's submissive to bigger dogs, it could be just him thinking he's the alpha around smaller dogs.