How come?  I have never spent much time snow wheelin, how does rim size affect snow traction?

because the wider your wheel is the more your tire is able to flex out when its aired down.  The wider you can get you tires in the snow the less you sink which lets you float on top of lots of snow.  Up in Washington most everyone loves snow wheeling and guys up here run wheels as wide as 15x15's.  I know it sounds crazy but they run bead locks and run around 1-2 psi.  My 37x14.50x15 toyo mt's are mounted on 15x12 MT Classic II's.  I usually run around 2-4 PSI in my tires. They are siped all the way across the tread to help me get better traction when the snow is heavy, wet and becomes really slick when its compacted.  Snow wheeling can be a lot of fun but you have to have soft tires and not be afraid to air down.  BTW I don't run beadlocks either and I've never lost a bead.  Its all about the right tire/wheel combo.  IROKS are awesome in the snow too.