Nothing to do with rear seat comfort, it's more to do with the mechanical bits. A Superb or Leon will typically be Front Wheel Drive based, a C Class or 3 Series will be Real Wheel Drive based, and an A4 is likely to have all the Quattro stuff at the back as well, which eats up boot space. Even a 4wd Superb or Leon will have less mechanical stuff at the back, due to the differences between Quattro 4wd systems and the Haldex stuff in Superbs and Leons.
I'm actually quite lucky with my 1 Series, as even though it's Rear Wheel Drive, the saloon style boot means theres a nice wide space behind the suspension turrets etc. If I had a similar generation 1 Series Hatchback, I'd be taking the woods out and struggling to fit my trolley in as well. As it is I have the best of both worlds - a nice golf club friendly boot and drive to the car comes from the correct end 🙂