Backup Size discrepency

Hi,

I came across a strange issue regarding DB backup size.If I run the Standard Report from SQL Server Studio it gives me the size as 139 GB but the actual file size is of 53 GB.

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How ever in DB02 it shows the size same as the standard report.But if the physical size of the file it is 55 GB.

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I know that we are taking the compressed backup.Here my question how is the standard report and DB02 calcualting the backup size? Does it mean that DB02/Standard SQL Report is giving us the uncompressed backup size? Any documentation to confirm this please?

Thanks,

Mofizur