This is now at least two data points I've noted in which it seems that the factory resonator actually does quite a bit of sound attenuating. I was always surprised at Big's results and now I'm convinced it's due in part to the OE resonator. I'd be willing to bet that Big's setup is a bit quieter than PatsFan's because aftermarket resonators don't seem to do a whole-whole lot, typically.
Here is a photo of the inside of a 330i resonator, and you can see that its design is quite a bit different than that of, say, a Vibrant resonator. More room (volume), lots of 'packing' (already removed before taking the above photo), and also, depending on if those two 'chamber walls' are perforated, something must be going on in there to deaden the sound. Who knows - I'm on fluid dynamics engineer, but perhaps the fact that the resonator is JUST after the Y merge has something to do with it, as well.
OEM:
Aftermarket (typical):
