Beemer E91 owner here. As others have said - seats up the space is not vast.
However where you win (other than the way it goes round corners ) is seats down. The rear passenger area is pretty spacious - there's loads of legroom compared to my old B5 Passat, so when the seats are folded you get a pretty decent space. Add in that the seat squabs don't need folding up against the back of the front seats, and the internal length is a hair longer than the Passat. Max width is compromised by the wheelarch intrusion, but it's not a massive problem with bikes.
We can comfortably get 2 people, 3 bikes and a fortnight's worth of kit in, and not be over the level of the luggage cover (ie still full view out of the rear view mirror).
As an alternative - have a look at the Jag X-type. Being based on a Mondeo floorpan, it's pretty big, although the floor is still fairly high as most of the petrol models are 4x4, so need space for a rear diff. Not as nicely made as the german stuff, but very viable.