I've been using claws-mail since well before it was claws-mail (back in the sylpheed days).
When I checked for new mail or opened dialogs they opened in reduced size. I upgraded to stretch a couple of months ago, and to buster just last night. Since the stretch upgrade all claws-mail dialogs opened in maximum size. If I catch "Get mail" before it's finished, I can reduce the dialog to a business-card-sized window, and that works for the current session, but if I exit the program the dialogs return to full size whenever I re-start.