So, you have the KAMEI Corvara 310 which 310 liters, and the Delhin 380 is 380 liters in capacity. i.e the number is the capacity.
Does that mean the Thule Motion 800 is 800 liters? I can't seem to find the literage, but that does seem like a lot of storage - more than twice as much as the 310. It doesn't look that big from the pictures.
I may as well go for the biggest possible now I know the Motion 800 looks OK on an E90 (I've also now sorted out somewhere to store it).