Who out there browses to a size, resizes there window, then browses to another website, then resizes again and so on? That makes no sense.
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.
Probably not a super common scenario, but not ultra rare either.
I keep a browser window open at all times. It is never full screen, because if it were full screen I wouldn't be able to see multiple windows at the same time.
I keep my browsing window as close to 1024x768 as possible. In 2019, a lot of websites can't handle a browser window using a mere 75% of the laptop screen, so they either render incorrectly or, worse, switch to a mobile view. When that happens, I either blacklist the website forever in a contemptuous fervor, or just resize the window. Apparently, this resizing action is trackable.
When I say "as close to 1024x768" as possible, I mean exactly 1024x768 unless I have resized it and forgotten. I use a little AppleScript thing to resize it to 1024x768, precisely for browser fingerprinting reasons. When you resize the window by hand, you typically end up with a VERY unique window dimension.