I'm one of those non-fullscreen browser (and most things in general, other than vim) window people.
> Having a large window should be telling the site "you have this space, make the best of it"
I 100% agree but I interpret this in reverse: if I made my browser window gigantic I expect you to use it. If I make my browser window small I expect you to responsively shrink.
The reason this doesn't happen is that it's pretty hard to build layouts that work across all these different screen sizes. That's generally why you get mobile/tablet/1600x900 desktop layouts and that's it.
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I suspect this is another consequence of the app vs. text tension on the web. Apps are great to fullscreen: you want Spotify or Figma to take up the whole window. You don't want a single sentence to unfurl entirely horizontally. But there's no way to say, "I'm the kind of user who sets the browser window size to as much as you can use, so use it all" or "I'm the kind of user who wants you to center your text elements and take up at most 1600px of space even when my browser window is using 3800px."
