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How does SQL Server calculate the initial size of a (compressed) backup?
For compressed backup SQL server take the 1/3 size of the database. That can be vary once than final size.
for more details following links might be helpfull:
https://blogs.msdn.microsoft.com/psssql/2011/08/11/how-compressed-is-your-backup/
https://blogs.msdn.microsoft.com/psssql/2011/08/11/how-compressed-is-your-backup/