This is a suprising element. Yes, a smaller wheel will have less moment of inertia, less weight, less aero drag on the tire, more passenger space and a smaller opening. ...But typically car makers design their cars to understeer so that impacts happen from the front. Smaller wheels in the back would make it oversteer-capable... which could be a big advantage selling it to the racing comunity
You'll see the production model come out with front tires 2" smaller... prototypes always show up with HUGE rims and sell with regular sized ones.