I don't think it turns out this way at all, since I don't recall ever having the option of paying for more room (and nothing else). The physical seat type alway

I don't think it turns out this way at all, since I don't recall ever having the option of paying for more room (and nothing else). The physical seat type always seems to be bundled with something else, such as an upgraded meal. Many airlines now have a "premium economy" section on international flights which offers just this. Maybe some of them have minor extra features too (like, I think I got a free drink before takeoff, and I certainly got better service, the one time I got upgraded to Premium Economy) but basically it's just the extra legroom. I'd probably pay for it on long flights if I were just a little bit richer.