Sizing down is for real - Ducay is on the right track, but it's not contact area.
Part of the winter tire's ability to grip is its soft compound and sidewall flex - downsizing gives you more sidewall, giving you more flex in the tire, as the manufacture can use a less stiff compound on the shoulder and edge of the tire to get the same weight limit the car needs, giving you improved ice turning abilities, all things equal.
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