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.

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.

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