That is correct. RawTherapee writes some info to the options file when it is closed, the window size at the time of closing is one of them.
You posted in the correct section, added info and OS in the title and this topic has been pushed to the top multiple times over the last few days, not much more can be done. Maybe someone will pick it up after the weekend.
Assuming you haven’t already: Might want to ask on a MacOS specific board about this behaviour.