I'd say that maximizing is more general then having a custom sized window. There are only so many resolutions in use (and I would guess 50%+ is either hd, full hd or qhd). Resizing your windows yield nearly unlimited options for browser sizes.
It's a question of consistency. A few version numbers + window sizes are going to stay the same every visit.
It only takes 33 bits of information to uniquely identify everyone on the planet and you likely can get more than 1 from a maximized window.
