I would argue that it's more a factor of cost than size. Each segment contains vehicles of a variety of sizes, in the Oberklasse you have everything from 2 doo

I would argue that it's more a factor of cost than size. Each segment contains vehicles of a variety of sizes, in the Oberklasse you have everything from 2 door supercar up to large SUVs. The guidelines for the Fahrzeugsegment by the Kraftfahrt-Bundesamt are really kind of crap. If you gave different people the Abgrenzungskriterien to classify all the cars, you'd end up with different results unless because there's an unspoken weight to the criteria. If I had to hazard a guess, probably 90% of the weight is on base price and another 9% is on "is this vehicle expensive because it's a Camper or Work Truck".