Okay, just checked with one of the dialogs here. If you double-click on the title bar it toggles between maximized and restored. Also if you clicked on the O icon in the top corner it would display the control menu (same as Alt+- on the keyboard) which shows Maximize, Restore, Move and Close - though it is already maximized and can't be moved while maximized.
So just double-click the blue title bar, or click on the O and select Restore.