Evidence for "narrower is better" - there's anecdotal such as at
http://forums.audiworld.com/archive/index.php/t-354407.html , recommendations from tire companies such as at
http://www.talontire.com/wintertiretips.html , the explaination at
http://www.f150online.com/forums/wheels-tires-brakes/51652-tire-width-snow-ice.html ( the narrow width cuts through the slush down to the pavement better being narrow - and despite what MalibuKen says, typically, for X PSI to hold the car up, there's the same amount of rubber - but is the contact patch long and skinny or short and wide? Still the same rubber on the ground), notes at TireRack's forum at
http://forums.nasioc.com/forums/showthread.php?t=1126453 - oh, several sources.
Now for the logic behind it.
When driving on dry pavement, you want a square or slightly wider than square tire patch so that you have better traction (more exposed) for maximum grip during heavy acceleration and decelleration. The reason is so that there's more rubber gripping and less obscured by the front rubber sliding (which can cause a loss of friction of the surface the tire's just deposited its rubber on, so the tire behind the initial patch will glide over the shed rubber compound ... )
Now, for SNOW tires - it's a different matter. First off, the rubber needs to stay flexible and pliable in colder weather. This usually means it won't last as long.
But to maximize the benefits of the weight of the car, a long skinny patch will tend to help cut through the surface slush and glaze with the weight of the car, allowing the rubber to actually GET to the pavement instead of gliding on the surface glaze. It does this by having the car's weight on a narrower patch - no, it doesn't increase the actual PSI of the rubber contacting the road, since the rubber patch AREA is the same, but the longer length means more weight goes over any particular unit of width than with a wide tire.
(Huh? Well, two ton vehicle, each tire is supporting about 1,000 pounds. If the tire is 10 inches wide, each inch of width supports 100 pounds per tire. If it's 5 inches wide on the ground, each inch of width supports 200 pounds per tire ... and as the car is rolling, that increased weight will help cut down through the slush.) (Note: Sizes and weights picked for easy math. Feel free to do it with the actual numbers.)
Driven - was that factual enough? If not, let me know - I'll dig up SAE studies if you wish. They're out there.
RwP