RTS maps are meant to be large. FPS maps are small so you can cover ground a lot easier. This is therefore a breakthrough excluding the Battlefield series.

RTS maps might give you maps on a miles and kilometers scale, but you dont experience it at the individual level.

FPS games get you to that individual level, or at most--a squad level.

anyone rember Soldner:secret wars? Those maps were pretty big, too bad that game sucked.

and with supreme commander, the size dosent mean all that much to me because nothing in that game is smaller than a skyscraper. And there isn't much to show its scale, you can cross those maps in secs. in crysis it might take a bit longer. (thinking of large map sizes, I just rembered Planetside)