Generally this indicates corrupted files of some sort. It's normal to have a few hundred megabytes, but gigs worth indicates a problem. The way to fix this is restore your iPod in iTunes. Plug your iPod in, let it start iTunes and sync (if that is how you have setup), and on the Summary page that appears after a sync, select Restore.
You are going to be asked at least one question during the process - Restore from backup? In this case, say yes. If the 1.6GB is still there after the restore, you are going to have to do the same thing, but say "no" to the question and it will restore it as if it is a new iPod.