Having designed and built the suspension on Formula-style racecar and dealt with tire loading at 1.6 lateral g’s, slip angles, and so on… please hear me out.
1 - Contact patch size vs tire width. There is absolutely no correlation between the two and comes down to tire edge design. I’ll take my dad’s car as an example, a 1997 Jetta with 195/60-14 tires in both summer and winter. However, his winter tires have a wider contact patch. Confused, yet? Tire width is dependent on the underlying structure of the tire and measured approximately where the tread meets the sidewall, so yes, that is standardized. Edge block shape then determines the contact patch size. My dad runs on Continental all-season tires due to his relatively high annual mileage that have a significantly rounded edge to the tire. His winter Yokohama Ice Guard have square edges on the tread. Patch size does change longitudinally a bit with pressure, but laterally, no. If anything you change the pressure distribution along the entire surface of the contact patch and may screw up tire wear (bad alignment will also do this).
2 – Edge shape vs tire style. This almost wholly comes down to performance criteria. Most performance tires will have a square shape to them to maximize the contact patch, and consequently the max grip. The same applies with winter tires and specific requirements for them… the sharper edge is to provide ‘bite’ and improve cornering grip, while a rounded edge almost acts like a ball bearing and just floats in slippery conditions.
3 – Rubber compounds differ from tire to tire. From hardest to softest, all-season/performance summer/winter tire. As it gets colder, tires become harder and will not be able to conform to road surfaces as easily, therefore less grip. Also, venturing in the chemical side of things, winter tire compounds are built on the multi-cell concept where the tire absorbs water(snow) on a microscopic level to allow more rubber to contact solid ground.
4 – Tread design and the misnomer of M+S tires. Yes, M+S is complete B+S… my high-perf summer tires are rated M+S. This a rating based on the % of rubber surface area vs contact patch surface area at the manufacturer’s recommended tire pressure and NOT rubber compounds. Most winter tires will have more tread blocks with sharper edges, again for more ‘bite’. Wider spacing of the blocks allow the snow and ice to be thrown out more easily.
5 – Tire width vs ice/snow. Same concept as skiing… wider floats, but in this case it is bad. Narrower is better in snow as it cuts through the softer snow on top, so that it can reach the hard pack below where there is more grip. Wider tires are better for ice use as there is less pressure and more rubber to grip it. On a microscopic level, more pressure results in ice melting and creating a film of water between rubber and ice, making it even more slippery. As a result, tire companies have created the multi-cell compounds to absorb this water. Don’t bring WRC winter tires into this… they are ultra narrow to cut through the snow in order to reach their high-grip surface – ice. Yes, ice. What you don’t see in that pic are the 200+ (on each tire) quarter-inch long metal studs that actually dig into the ice below and chew it up. Rally Sweden is considered as the Rally with the most grip… go figure.
What does this mean for the original poster? Minus-sizing is fine for winter use. I had 185/65-14 Winterforces on my Integra and plan on using the same set on my Jetta. Cheap, good in the snow, ok on ice and As long as it’s a true winter tire, you’re better off (and safer) than before. Also, with a light Japanese car, throw in a 30-50lb sandbag to prevent the rear from floating due to lack of weight.