GROOM should be run on target restore database as well as the source database.
Why is my restored database much larger than my source database?
nzbackup and nzrestore capture deleted records from the source and apply them to the target. However, if one GROOMs the source database after backups, the space occupied by the deleted records is cleaned up there, but is not applied to the target automatically.
If you see a large discrepancy between the sizes of the source and target databases, use a GROOM RECORDS ALL on the target, then re-compare them.
Note: A small discrepancy is normal when using compression, or when the source and target systems are different sizes (ex. 3 SPU system and a 12 SPU system).