I have dozens of webpages open in my browser window. The tabbed UI means ALL webpages have the same window size. If I were to resize my window for various sites (or even various locations on the same site or page) I'd do nothing else but resize. Just switching back and forth between tabs then often requires resizing the window! I think the idea of "make websites full width, users can resize their window" builds on a few assumptions that were perhaps true on a linux desktop in 1996 but they aren't anymore:

- Users have reasonable window managers for non-maximized windows(they don't, they run windows mostly).

- Websites display text mostly that can flow nicely.

- Web browsers have one window per web page