Actually no, both parts need the same cell size because you will get complex flow structures due to the interaction between the air and the car. If you take a look at the mesh settings, you will see that both parts (upper and lower) have the same refinement level (i.e. same element size). This is done to observe the flow more accurately around and behind the car.
The splitting of the refinement regions is done to optimize the mesh size. I hope this picture can illustrate what I mean:
Hope this helps