really take the time and do a omnidbutil -purge -filenames -force and try an export/import afterwards.
This will free up a little more space than just the purge, takes a lot longer of course.
With such a big IDB I also suggest checking your Log-Levels. If you use LogAll go down to LogFiles and reduce the Filenames-Part to 1/3. omnidbutil -list_large_directories can also help identifiying Mountpoints with to many Detail-Info, setting some of them to LogDir (where possible) can also save some space in future Backups.