The Caching system in UBB.Classic is one large thing that can effect the size of your site on backup time (noncgi/cache-[8characters]/). The size of the cache will grow based on users activity on the site as every "view" of a page that hasn't been cached (for instance, if you clear your cache and click forum 1 and the first topic, that topic will be cached so that others can view it faster; when it's updated (responded to) the cache will be updated).
The size of the cache can really vary dependant on how popular a forum is as well as how often the cache is cleared; the cache (I believe) can get to the same size of your forum (forum size based off of the "forum" directories in your noncgi directory).
