Um, I'm not saying that anyone is wrong in what they have said, however I always thought that, for snow at least, you wanted a skinny tire to reduce your lbs per square inch to sink into the snow better, thus getting to the surface of the roadway more, providing traction.
I guess it could be true that with wider tires, you would ride on top of snow, but it has been my experience that for snow you should have as skinny a tire as possible. I also heard that with wider tires, you actually plow the snow as you drive causing the drive tire to push more snow.
These are by no means facts or bible truths, just my 2-3 cents.