In the simplest terms, tire size relates to two factors - weight distribution (front/rear) and power delivery (which actually affects weight distribution under acceleration).
Lower horsepower cars with neutral weight distribution (or more weight in front - e.g., front-engined cars) tend to have even front/rear sizes. Although as the power goes way up (think 928's), you need more rubber in the rear to avoid excess wheelspin under acceleration.
911's, with lots of rear weight bias and lots of power, need more rubber at the rear, or you would have huge wheelspin and power-on oversteer problems.
Boxsters have very neutral weight distribution (slightly toward the rear), but have enough power that the rear tires are a bit larger than the fronts.
If someone built a front-engined, front-wheel-drive car with a heavy front end, you would probably see them running larger front tires than rear tires.
There is a cute physics problem which illustrates some of weight distribution effects - determine the weight of a car using just a tire pressure gauge, a piece of paper, a ruler, and a pencil.
Solution: roll a front tire onto the paper, mark the tire patch on the paper with the pencil, compute the area using the ruler (resulting in sq. inches), take the tire pressure (psi or lbs./sq. inch), multiply the tire pressure times the tire patch area to get the weight of the car on that wheel (lbs.). Multiplying that weight by 4 gives a pretty reasonable estimate of the car weight. If the front/rear sizes are significantly different (or have significantly different pressures), then repeat with the rear tires and multiply the front by 2 and the rear by 2 and add the results to get a better estimate.
Now picture a 2400 lb. car with even weight distribution on identical front/rear tires - each tire is holding 600 lbs. If the tire pressures are 30 psi, then each tire has a contact patch of 20 sq. inches.
If that same car has a front/rear weight distribution of 40/60 (40% of the weight in the front, 60% in the rear [just like a 993]), then the front tires are only supporting about 480 lbs. each, and the rear tires are each supporting about 720 lbs. The rears are now supporting 50% more weight than the fronts. If the tires are still the same sizes and same pressures (30 psi), then somehow the contact patch of the rears has to increase from 20 sq. inches to 24 sq. inches, while the fronts decrease from 20 to 16 sq. inches. This would produce much more wear on the rears as they are now deforming much more than the fronts as they roll along (in order to maintain that greater contact patch).
Increasing the width of the rear tires in this case allows increasing the contact patch without forcing the tire deformation. If the original 20 sq. inch patch was 6" wide by 3.333" front-to-rear, then increasing the tire width to 7.2" would allow the same front-to-rear patch depth at the same pressure.
In practice, as the rear tires increase in size the pressures are usually increased, so in this case you might use a 6.5" wide tire with a tire pressure of 33 psi to achieve the same rolling profile. (You also typically see a drop in pressure on the fronts, to increase that contact patch.)
On the 951, the weight distribution is nearly a perfect 50/50, but if it is boosted to nearly 600 HP (a Las Vegas one claimed 564 HP and 560 ft-lbs of torque!) the standard tires just aren't going to grip the road when you mash the accelerator.
(Remember your PCA-SDR Driving School classes - when you accelerate, weight transfers to the rear.)
With that much punch, you probably get something close to 100% of the weight at the rear under full acceleration, so unless the tire width is nearly 2x stock, it is either going to deform a lot or (more likely) just spin and smoke.
(This is one reason why dragsters use rear tires that are 17 inches wide and have 10 inch front-to-rear contact depth, for a contact patch of 170 sq. inches. Plus, there is a lot of science in how those tires change chemically and physically as they burnout and then race.)
Gosh, I knew that Physics degree would come in handy some day.
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