So, I made a little test here.... I clicked the button and the window appeared enlarged (like maximized).. Then I made it little and close it... Next time a clicked the button, the window opened little as I have made it (and in the same position). So I enlarged again (I didn't use the maximize button in any time!!!) and close it, the window opened "big" again...
So you have to enlarge the window that opens when you click the mail button....
Note that if you maximize the "little" window opened with the button, next time you open it, it will be little again...
/ 
