[beatgammit]

Even if they do, there's variation in what "full screen" means. Some people have the bookmarks toolbar enabled, others don't. Some have compact icons, others don't. Some keep a sidebar open, others don't. Some use a theme that changes the size of toolbars. Some have a dock/taskbar always visible, some have differently sized taskbars, etc.

This all leads to a huge variation between users of even the same screen size (e.g. 1920x1080), since the portion of the screen available to the page is different.

The Tor browser fixes this by having the window always be the same size on all machines, regardless of screen resolution. This is a bit annoying because it means you have less stuff on the page visible at a time, but since it makes you look the same as every other user, it's worth it for privacy conscious users.