Ross, it sounds like you have several backup copies, which is great. If you present Project is backed up I would then run "Pack" which will clean the database of old records. Go to Utilities > Project > Pack. After this runs make another backup .sqz file and see what happens to the size.
Not sure why the growth in the file size though. I can see it getting smaller if you unchecked a box on items to save, unless you may have added one of the items.