Sorry to destroy your illusions, but here in Europe, the standard wheel size on the Audi A8, BMW 7-series as well as Mercedes S-Class is indeed 17 inch! You can get 18 to 20 inch optional wheels, but up to now I have never seen cars outside the SUV market with 20 inch wheels or higher. Remember, most people who buy luxury vehicles like the abovementioned three over here seem to prefer luxurious ride quality over looks.
By the way, cars like the Golf are not considered "small" here, cars like the VW up! or Smart are. Their wheels start at 14 inch, the average Golf has standard 15 or 16 inch, depending on the type. The top of the line GTI has standard 17 inch wheels.
The "large" category here starts with cars like the VW Passat, Audi A6 etc.
I think there will be many people here who like to buy cars the size of a Model S but who will be put off by the thought of the costs associated with 19 or even 21 inch wheels.
Most tire dealers here don't even offer 21 inch tires or only by (extra expensive) special order. Even 19 inch tires are quite hard to come by here, there just isn't much demand.