How "open world" is it really, though? If TPP was split up as a bunch of separate areas that were GZ camp in size, or bigger, is that different than the areas in MGS4, for example?
"Open areas" is probably more accurate. Trying to dig it up now, but Kojima said that apparently a lot of areas are much bigger than the GZ map. I think when it was cited as open world, it was because you could do a lot more like drive or do some side-missions before continuing with the story. I'm assuming side-missions are given in-game rather than picked from the menu in TPP.