actually the save games do not store the map in them at all. they are basically written in the same form as the .dll files of each mission.
in truth when you load a game, you are just calling up scripts from the save game file in the same way as if you were initiating a new game.
it doesn't store the map itself, what it does store is unit locations, current tasks/commands, possibly some pathfinding information, the time count, and any disasters (and their current attributes).
basically, like i said, its as if you are loading a mission, cos that is how i assume the game looks at it. (missions can start with units in patrol paths, and with structures already damaged... and probably with disasters already in effect, i wouldn't know about that last one though... unless you count a blight-countdown-till-arrival in the missions as a sort of disaster already running at the start... but then, is it a countdown, or is it a script: blight appear at (x,y) at count 2000?)
[size=8]edit: basically when we have a mission editor, we'll not be far from a save game editor[/size]