Click the text at the bottom of the table or press (t) to compact the size of gmail.
1. "It works (compacting)", but:
2. Unobvious: It toggles compacted <-> uncompacted when you click anywhere in the read message area (any blank space, other than links).
3. Confusing/Distracting: When you click at any email (in the email list area), that clicking at an email toggles compacted <-> uncompacted, too. That moves/shakes the screen - very unpleasant!
Works (Firefox 114, ViolentMonkey). I can't locate "the text at the bottom of the table"; but pressing (t) key does work.