Before you stat fiddling in the profile folder.

File menu (press F10 if the menu file is not on the top of the window) Compact folders that frees space, it is a housekeeping function. BTW that file is the outbox folder in local folders.

I have compacted folders using the File/Compact Folders/ option but it does not empty "unsent messages" in: c:\users\comdata\application data\thunderbird\profiles\jnuthszm.default\mail\local folders\unsent messages which still has 934MB. Why?

In thunderbird go to local folders, Outbox. R+click and pick properties. Does size show as roughly 900mb? If it doesn, then you want to compact that folder

Selecting Outbox, Properties showed 934MB and I activated the option available there "Repair Folder" which completely cleared the Outbox. Thank you.