It's not an unfair advantage because you set your resolution differently, it's an unfair advantage because you can see your enemies approach earlier than they can see you. For this reason, JJ2+ leaves you with a black border so that you can only see 640x480 of the level (if the server uses standard settings). In other words, using bigjazz as is along with up-to-date JJ2+ isn't considered an unfair advantage