Right click on your packages folder, and select properties. If you have the content from ISI, and some custom tracks there, it can reach around 3 Gb easy. And thats only the pacakges, so the installer files for the content.
The big question is, what do you call "resetting the game". Cleaning it up with custom tracks/mods, or resetting player data?
As Ross said, player info is stored in the player folder. That should not be that big in size.