I think I got it now. It was a setting, I think.
In the "Control Center" in the "Window Behavior" section, then the "Moving" tab, it says, "Allow moving or resizing of maximized windows" Then below that:
Placement: (it it now set to "Smart")
It was set to "Maximize" before. By the wording of that, who would think that this setting would affect Dialog Boxes?!
Anyway, so far, the dialog boxes are behaving nicely (the right size).