The default TOR viewport size isn't random. It picks a multiple of 200x100, with 1000x1000 as the maximum

I think the idea is preventing tracking a user between different Tor Browser sessions, not within the same session. When Tor Browser opens, it opens at a default size, regardless of computer. So there's no way to differentiate between different Tor Browser users who don't manually change the window size.

If your screen resolution is common, it's just a bit or two worth of information. But enough of those bits from various sources and you are unique.

> If you have a random window size you have a much higher chance of it being unique.

That's the whole point, multiple unique data points can whittle down the differences to a single user.