It's really impressive. I wondered exactly the same when i first installed the game: how on earth can it be so small compared to other HDD consuming games.
Well, I guess there is a very small amount of graphics stuff, textures and what have you. Roads are the same everywhere, cities look pretty much the same.
same dev made the 18 wheels of steel (18 WoS) series, back then they didn't use more than 200-300MB I think.. and the map then was even bigger, it was all of usa, and parts of canada and mexico
Repetition is indeed the trick they mastered there . By using the same textures and graphics over and over again, they make a sort of jigsaw puzzle of the map. So the map is infact a textual reference to (a relative few) graphics files.
There are a lot of cars around, but not many different ones. Saves in space as well.
It's just a case of decent engineering they did. Very nice job.