Last week I needed to use Uber, and my ride turned out to be a previous-generation Audi A4. I have a friend who has one (actually it's an S4, the sportier version, but it's the same car) but I'd never been in the back seat of his car. A couple of months ago I used Uber and my ride was driving a previous-generation BMW 3-Series sedan, a car I'd never been in before.
Both of these cars are often bought by image-conscious people. Neither car is particularly large, but when I use UberX the front passenger seat is usually moved all the way forward to maximize rear seat room for a single passenger. I found the BMW's back seat to be much more cramped and much less comfortable overall than the Audi's. This isn't exactly a surprise, but it was interesting for me to be able to confirm it personally.
I also wonder how the newer generations of these cars compare to each other. I don't think anyone buys either car because of its rear-seat room, but even if people aren't driving for Uber, they must have passengers in their back seats sometimes.