That's a big file for just two cars, certainly. Have a look at your sound files. There's a 137MB folder 'sfx-samples' in the gt3 download. Are these directly used by the car or were they for reference? I can't ever remember seeing a folder called this before, so don't think AC references it. I might be wrong though.
Even the standard 'sfx' folder for each car is 272MB, which seem big. There are two sound banks in there, each taking half of that 272MB.
Maybe some redundant files in there?
Skin folders are also quite large.
Edit: nice job with the cars, btw..!