The Help Vault doesn’t expose this behaviour here.
Are you by any chance using @Lithou’s Image Flags Snippet?
In my vault, the snippet is called pub-Image Flags.css. This is what sets the table width to 100% and its layout to fixed, in my case.
Anyway, I’m working on a CSS snippet that will undo this change and make tables have self-adjusting column width again.
Link will be up here in a few moments, after testing.
EDIT: Here’s the link → Meta Post - Common CSS Hacks - #417 by Moonbase59