http://www.steelpanthersworldatwar.com/main.asp
Only 425Mb but runs under all Win OS's. Not sure about other OS's. Matrix Games has permission to tweak the code and distribute but can't sell it. So instead the make and sell "Mega Campaign" add-ons. Matrix and the SP community has put a lot of work into this game and it's still being updated. If you can handle the download this the version to get.
http://www.matrixgames.com