Not sure if this is a RawTherapee issue or a MacOS one (mind you: I’m not a MacOS user).
If I look at your screenshot I notice that the window created by MacOS does seem to be large (almost full screen) and that RawTherapee is small inside that dedicated window. I’m not aware of that being an option that MacOS offers (based on some on-line searching and digging).
Is this issue restricted to RawTherapee or do other applications also show this?
Either way you might want to also have a look at your MacOS preference settings (possibly the ones that are RawTherapee specific).
The configuration file (called options) should, on a MacOS based system, reside in:
~/Library/Application Support/RawTherapee/config/
Assuming you found it: Rename it to options_org or something similar and start RawTherapee.
Restore the saved file if it doesn’t solve the problem (this will give you your personalized configuration settings back).
Just in case: Leaving the options file as-is and removing the [GUI] section in that file (on my machine that would be up to and not including [Crop Settings]) would probably suffice. This would keep all your other configuration settings as they are.