More to do with the width of the tyres on BMW's imo, they're usually quite wide compared to other marques which doesn't help in snow. When my rear tyre blew out recently, I had more grip with the spare wheel on the back which had a much smaller contact patch to cut through the snow.
Obviously brimmed tank and lots of ballast in the boot will help loads. I actually used my rather hefty brother yesterday to sit in my boot to get me going through the thicker stuff.
Y'know a thinner tyre cuts through the snow better because it puts the pressure on a smaller contact patch? Well why does letting some air out of your tyres to get a better grip in snow work then? Surely you're trying to get a bigger area of contact when you're doing that?
Yes and no.
A 'properly' inflated tyre is not good for traction when on ice, snow, wet or even dry roads. A softer tyre will give much better traction in all conditions, but at the cost of wear, especially on dry roads.
When they find a way of making side walls as sturdy as the contact surface without reducing ride quality, we'll all be driving around with 10 PSI in our tyres.