There are two school of thoughts on tire width and performance on tire/snow.
On stuff thats not too deep, still fresh (as in not too many vehicles have gone over the terrain), and slow travel speeds - most would argue that tires with smaller widths are better because they dig through the snow/mud and grip at the stuff underneath which is usually more firm. For high speeds, you will eventually "float" on top of the mud/snow and at that point, greater surface area, in other words wider tires, are better because there is less pressure per square inch on youre tires.
I mentioned the "still fresh" idea because, as more cars run over snow, it tends to be compacted and compacted so that over time, it's actually ice at the bottom of the snow.. so no matter how many pounds per square inch you have, you're still riding on ice.