Your expectation is correct, application size should not differ by much. But there are several variations in the product (Setting to retain deleted files, Retaining multiple versions of files, Retaining files to cover stubs copied outside subclient content, etc.) that can result in application size not matching size on disk of client computer. So without looking into details, it is very difficullt to comment further.
Can you please escalate the issue through Commvault Support?