I've never seen this before. I have an 11GB backup file(native SQL compression). I restore it to a test database which has a used portion of the data file of around 50GB(the file is actually 75GB buta thrd unused). The immediately backup the file again and it backs up to a file of around 9GB.
How can this be?
Some other info:
The database is backed up and restored to the same sql instance.
No SQL service packs or patches applied.
No Windows patches applied(although the server WAS restarted between the backup and restore, so potentially something was left unapplied)
The backup was taken at the start of a failed vendor deployment, and I don't know what else they did.
Backup has no filestream files, but filestream is enabled on server.