Size doesn't matter
It's all about heat. Wider / larger tires don't overheat as easily as narrower / smaller ones. Therefore you use wider tires on axle that has more load and on drive axle. In case of cars that have weight distribution towards drive axle, you get even more disproportion in tire size.
Simply put - it's tightly related to how much work tire is doing. Work produces heat and larger surface helps distributing that head, so temperature remains in optimal range.
Modern open wheel cars have high power-to weight ratio, they're rear drive and they have weight distribution towards rear. This is why we see biggest disproportion in tire size with such cars.
Of course there are some influences on "quality" of contact path, so there is still some positive influence on grip. But it's not the main reason why some tires are wider.