Actually it probably will be.
That query returns the total size of the DB files. A backup does not back up the entire file, just the used portion of it. So if you have a 10GB database that's only 50% allocated, the backup will (ignoring contribution from the log) be around 5GB in size.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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