GT2 is too big to fit on a single disc because of the size of the intro movie and the many GT mode menu backgrounds, which is why it was split into two discs and the intro movie isn't on the same disc as those menus. There's actually a decent bit of space left on the disc, and because it's a PS1 game all of the assets are tiny.
For example, the NSX-R GT1 Turbo added in the latest update takes up 34KB on the disc for its model and textures, 12KB for its logo in the dealership / garage, plus another handful of KB for its physics data and dealership menu page. In short, each completely new car generally fits well within 100KB of disc space, and restored cars that already existed on the disc in some form such as the Ruf CR2 only need the physics data. The limit on the number of cars is actually the number of files that the game can read from a single directory, and how much space is allocated in RAM for the physics and upgrade data.
On top of that, the efficiency of the deflate compression that the game uses extensively has been improved massively over the last 20 years, so the parts of the game that are recompressed are smaller than they originally were, although I do this as little as possible to avoid increasing the size of the patch download. I've also gotten rid of some leftover junk on the disc, and rebuilt the content for the JP and US versions based on the final PAL release.
Regarding Vauxhall, both the Vauxhall and Opel dealerships are present in North City at the same time, in all languages and regions, and Acura is in South City. The added Vectra is an earlier 2.0 SRi version that was replaced close to release by the 2.5 GSi, but I've only included the Vauxhall SRi as the Opel equivalent doesn't really match a real specification very well.