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.
How so? Wouldn't there just be a one to one mapping between resolution and maximized windows size? The only way to prevent this is to not maximize windows and instead have custom sized ones, but that just makes identifying you even easier.
The laptop I'm using right now has a 1366x768 screen, a maximized browser window is 1306x768. I bet many other users of of that size are going to have browser windows that have the full 1366 px width, but then some height <768 px. Details depend on the OS, the size and position of the taskbar/dock/whatever, ... It's not much, but it adds a bit or two of data.
Custom sized ones make you easier to identify only if you don't change them.