I don’t know the actual mechanics of the game save file, but as an IT person who deals with cloud services, databases, etc., for a paycheck, it seems to me things can be decoupled from your game save file to make the actual file smaller by storing assets like cars, tunes, and paints in centralized cloud storage with IDs, and in the case of tunes/paints a bit for whether they’re shared or not, then your game save is nothing more than a list of lookup values as to cars you own, career races completed, tunes/paints you created/downloaded, tied to your unique ID.
$0.02