When backing up MS SQL database, OBRM/SOS Business backup progress often shows smaller size of the database being backed up (db size = 5.63 MB, backed up size = 2.30 MB):
This is normal situation and the reasons are following:
- LDF is not used during the backup.
- free pages in the MDF file are ignored. So, backed up file becomes smaller than the actual MDF.
More information can be found at: http://dba.stackexchange.com/questions/3694/why-is-a-bak-so-much-smaller-than-the-database-its-a-backup-of
When a DB is created, you can specify (for performance) how much space you want to allocate to the data and log files. This space is then reserved even if no data is stored in the tables. Only the extents that have data written to it are backed up. In your case, your MDF/LDF total could have even been 100 GB but your backup would still be around 23 GB for the backup that you did. If around 1 GB of data was added, your MDF/LDF total would still be 100 GB, but your backup would now be around 24 GB. A full backup contains all the extents that have data in them and a bit of the log file. The full backup contains all the data from the time the backup task ended, and not just from the time the backup task started; this is why a bit of the log file is also required.