There is another thing to consider regarding folder sizes:
Everything scans local NTFS volumes on a low level and therefor 'sees' more files and fodlers than a filemanager does, for example the Home directory of other users and some Windows folders.
Those will be included in the reported folder sizes.
(FWIW: These files will not be reported in search results as Opus will remove them from the list during 'post-processing' for showing in Lister).
My personal opinion regarding folder sizes:
Folder sizes reported by any tool should be used as just a rough indication. Modern filesystems got too complicated to give correct results.
Some examples:
- No tool I know of takes Alternate (/Named) Data Streams (ADS) into account. A 1KB file can have several 1GB datastreams. Reported size will still be 1KB
- Should Directory Symbolic Links and Folder Junctions be "followed" or not? One day you would want it to, the other day you won't.
- Hardlinks: a file can have multiple names (like C:\Windows\System32\notepad.exe and C:\Windows\notepad.exe) while being stored on disc only once. What size should be reported for C:\windows?
(Fun fact: Different Windows versions report different Size on Disk values for the exact same file ...)
I think for a Folder Size column in Opus, a rough estimate is enough to get a quick impression. When you want exact values (OK, more or less; see above) press 'ALT + ENTER' on a folder and let Windows calculate what it thinks it is.