Your memory is correct Nicolai ...
You will need to reset the nbdb passwd first, as by default it is random
nbdb_admin -dba <new passwd> (can answer 'n' to the question)
A NBDB rebuild might bring the size down , will take a while though during which time you'll need to stop NBU and only have NBDB running.
It could be the case that one of the db tables is massive - I think one of the audit tables can be an issue. Not sure of anyway to find this other than to run :
/usr/openv/db/nbdb_unload </some output dir with 1TB space> and then see which .dat files are the biggest, then look these up in reload.sql to see what they are.
If i change nbdb password will it effect the backups?
actually we are planning to upgrade from 8.1.1 to 18.104.22.168 before that do i need to perform rebuild/reorganize of EMM if its size is more then 4-5GB?