Last year's IMAP mail I just finished archiving (before old mail is deleted by Apple). Though it's only text, 11.0.2 doesn't compress all that much: 333.7 MB of text was compressed to a 235.4 MB zip file: 29% 'disappeared'. Perhaps my expectations are too high, such as 50%.

The only good news is that a nifty icon with a zipper is hidden deep in /Library: Ctrl-click on a Stuffit zip file and open the info window. Click 'Open with' and drop down the menu: BOMArchivehelper is the default, and mine, on MacOX 10.4.11, is labeled 'BOMArchivehelper (default) (10.4.11)', suggesting this could be more than cosmetic. However, it compressed my mail to 235.4 MB, so maybe not.

However, I've always compressed using the Mac contextual menus and used BOMArchivehelper, its default, to expand. Never a problem here. Compresses & expands to the same folder. If you have this option without obtaining Stuffit yourself, at the expense of all your personal information, I should use it.