It's really crazy what camera angles and clever design can do. Check out GTA San Andreas for what I feel is the best explanation of this - the game felt enormous when I was a kid, but when you see the actual true size of that map, it's pretty damn small.
"Big" and "small" are funny terms when you talk about computer graphics. A surface 10 units long is short if your character moves at 10 units per second. It's long if your character moves at 1 unit per second.
You can easily see the entire GTA map and it might seem small, but if you place the camera at street level I think it's clear the map is rather big.
