1. Add and activate a custom column config with the following columns
1st column: [=tc.Size.Mbytes]
2nd column [=dirsizecalc.Size]
The order matters!
2. Open an archive
3. TC will now show the value in the same units in both columns although the dirsiecalc column is configured to show bytes.
Conclusion: TC forwards the value in the unit which is used in a column before the plug-in column instead of bytes as documented in the plug-in SDK.
Thanks to petermad for finding the bug
