Easy enough to reproduce. I am working on a 32 inch monitor fwiw, but the effect is the same on my laptop screen (though less prominent)
* Maximize Scrivener (I've tested this in both editor and outline mode) * Hit the button for composition mode. * Hit the escape button to leave composition (pressing the button on the bottom right has the same effect) * Behold that the your window is now confined to the upper left quadrant of your screen with a lot of white space around it.
Looks like the screen is drawn to the original non-maximized dimensions, but the window is still maximized.