When I'm using my macbook's screen, I usually expand a browser window to fill the whole screen -- it's a 13" screen so not using the whole thing makes things feel small. But most of the time my computer is plugged into an larger external monitor (20-something inches, maybe 27?), and there I don't expand any windows to fill the whole screen. I like having separate not-full-screen windows which partially (or mostly) overlap.

Somewhat relatedly, we use Windows at work, and it drives me crazy when I hop on a computer after someone's been using it and they have every single thing maximized, even Windows Explorer, on 27" monitors. A maximized browser, I get... I don't do it myself but I understand how it can be useful, but maximizing Windows Explorer is just insane to me, and yet a lot of my coworkers do it.