On second thought, I think it might just be the angle from which the models are photographed.
Looking at just the photos of the wheels and tires themselves, they look fine. Quite a few of the finished car shots are at a somewhat high angle- an angle you'd only see a 1:1 from if you were standing up on a stool or abnormally tall. From that perspective, the whitewalls look like they're tucked up into the fenders a bit more than they should be, which would create the illusion of a too-tall tire and/or an undersize wheel. Looking at the first shot of the white car, which is shot at an angle more like how you'd approach the car in 1:1, the tires look a lot better.
Might just be a 'perception thing' on my part.