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Hello!
I'm little bit confused with the functionality of these two options. When they both enabled, HQ videos shows in "fit to window", but HD videos show in a predefined sized player, smaller then "fit to window"...Perhaps it's intentionally, however, it just doesn't look right, when better quality videos displayed in a smaller player...

The way I think it works, is that 720p videos will appear at their TRUE size (1280x720), when that option is enabled. Normally, even in HD mode, youtube makes those videos smaller than they really are, unless u full screen. Note though, that not all HD videos are 720p, most aren't. And so without stretching them to fit window size, they look smaller than 'fit to window', while true 720p still look huge. This is because those HD videos weren't filmed quite as large as 720p videos are, But they ARE still considered HD.
Fit to Window makes HD videos, regardless of size, fit the window, and Big mode seems to make ALL videos big. I think.

I hope I've explained it properly/answered what you wanted to know. If not just say. lol