Used space: 54.9 GB
Free space: 864 GB
This morning's Avast scan shows - AMOUNT OF DATA TESTED - 120.2 GB.
Have you changed the default scan settings ?
I'm not entirely sure; what type of scan are you doing ?
Or is this just routine daily use of your computer and what avast scans ?
By default avast doesn't scan all archives (only specific ones - 3 - ), if you change that setting to scan all archives, then the archive is unpacked and the contents scanned. So the scanned data if for all content extracted and scanned and not just the size of the archive.
By default avast doesn't scan all files, if you change that to scan all files then you are going to get much more files scanned.