Brute force?

Get the size of every “file” in the volume along with the file id of each and then subtract the size of any files with a matching file id that are in the WinSxS. Then sum the size of the remaining files that were from the WinSxS.

You can get the file id using https://learn.microsoft.com/en-us/windows/win32/api/winbase/... or fsutil, etc.

You could also probably execute “fsutil hardlink list” for every “file” in WinSxS and then ignore any that list more than one result and sum the size of the remainder.

There are of course more efficient ways to do this but those are some quick hacks.

  dism.exe /Online /Cleanup-Image /AnalyzeComponentStore

Gives both apparent (seen in Explorer) and actual size.