Without the inverse table, TeenyURL will generate different teeny URLs for the same request. But if you enable the inverse table, TeenyURL will use more storage.
Use tracking allows you to purge older entries to save space. However, the tracking data itself takes up space.
URLs that haven't been used in this many days will be dropped. This involves a full scan so be patient. You also need to have Use Tracking enabled. 0 won't purge anything.