Here are the raw numbers including corrections:
530i (left number) vs. 330i (right number) and difference in parenthesis:
Front headroom (without moonroof) 38.7/38.4 (+.3)
Front headroom (with moonroof) 37.4/37.0 (+.4)
Rear headroom (without moonroof) 37.8/37.5 (+.3)
Rear headroom (with moonroof)37.2/37.4 (-.2) [advantage 330]
Shoulder room, front: 56.8 / 54.4 (+2.4)
Shoulder room, rear: 55.9 / 54.2 (+1.7)
Hiproom, front: 58.0 / 56.6 (+1.4)
Hiproom, rear: 58.5 / 56.6 (+1.9)
Legroom, front: 41.7 / 41.4 (+0.3)
Legroom, rear: 34.2 / 34.6 (-0.4) [advantage 330]
Passenger volume: 92 / 91 (+1)
Trunk volume: 11.1 / 10.7 (+0.4)
MY OBSERVATIONS:
The driver in the a 3-series with moonroof gets only 37 inches of headspace (even less than his passengers or the 5-series' passengers). Welcome to the 3-series, watch your head.
The 5-series (with moonroof) front seat occupants get 0.4 inches MORE headroom, 0.3 inches MORE legroom, 2.4 inches MORE shoulder room, and 1.4 inches MORE hiproom.
The 5-series (with moonroof) rear seat occupants get 0.2 inches MORE headroom than the 3-series driver, 1.7 inches MORE shoulder room, 1.9 inches MORE hiproom, and alas, 0.4 inches LESS legroom.
One missing dimension is the seat height. The 5-series interior is about 0.5 inches taller, which allows the seats to sit a little higher, and thus the passengers to sit more comfortably without bending at the knees (check it out). Get in the back seat of both, and you will be convinced that the rear seat passenger in a 5-series is much better off than either the driver or rear seat occupants of the 3-series.
Finally, unless they are very tall, most 5-series drivers don't position their seats all the way back, which means that rear passengers end-up with more space than the numbers show.
I think Phil puts it in the right context...if you crave better handling and lighter weight over extra space, the 3-series is a better choice.