SQL Server Backups suddenly change size Yesterday while I was traveling, one of our SQL Server 2016 servers choked on a TempDB that was full. Since I'm the only DBA, one of the system engineers tried to "fix" the server by rebooting.

This triggered some sort of iSCSI/SAN/LUN/etc. cascade of insanity that resulted in the entire data drive being lost (user DBs, system DBs, error log files... all gone). Their response was to supply me with a nice, new, iSCSI drive since they couldn't get the old one back. (sigh)

So today, I restored the system DBs from backup, then restored the user DBs from backup. I then did a full backup on all of the freshly restored databases. Just in case.

But I noticed something.

I restored a two-day-old backup of Master that was 785,489 KB. After completing all of the other restores, the new backup of Master is 718,519 KB.

I restored a two-day-old backup of MSDB that was 36,019 KB. After completing all of the other restores, the new backup of MSDB is 18,333 KB.

(I can't really do a similar comparison on user DBs because they were Full, Diff, and Log restores...)

The backups are compressed. These were full backups that I restored from, and full backups I created. As system databases, there are no diff or log backups being applied in between.

Is it normal for a just-restored database's backup to not be the same size or at least nearly the same size as the original?
A database backup contains the allocated extents plus the parts of the log required to replay any activity that occurred during the backup, and restore the database to the point-in-time at which the backup completed.

So your first backup file could contain more log records than your second backup.

David