nrv,
You appear to have a Dell computer that is apparently quite new.
I too have a Dell computer that is quite new.
I rather suspect that similar system images would have been placed by Dell on the hard drives of your system and mine. I know that my system arrived from Dell with less than 5Gb occupied on the C:\ drive.
There are certainly compressed files (including many CAB files) as part of the system image placed by Dell on the system. For example the C:\i386 folder occupies 340Mb of space on my system, when that folder is thoroughly scanned by avast then 1.3Gb of unpacked files is reported scanned.
It appears that you have added a large volume of data to the system. Only you know the numbers and mix of the files but if a large part of the extra data is Microsoft Office type files, Excel, Word, Powerpoint then I think you may indeed have a valid concern about the data scanned count.
My testing shows that when avast scans (at standard or thorough sensitivity) any of these document types the data scanned count is about double the real size of the document (I just scanned one single .doc file; the number of files scanned was reported as 10 and the scanned size was double the real file size). This seems to be consistent for any of the MSOffice type files (a folder with 25 documents when scanned reports 158 files and scanned count double the real file size).
When these documents are scanned at the quick scan sensitivity the scanned count is the same as the real document size and the file count is correct too.
So if you have 10Gb of MSOffice files then avast is likely to report it scanned 20Gb and very many more files than you really have. This might go a long way towards explaining the "overestimate" of scanned data you are seeing.
If you have a folder just containing MSOffice type documents you might want to just run a scan of that folder and see what the results show.