Right, the stock tires are 225/40 and 245/40, which means that their aspect ratios are the same, despite the fact they are two different widths, which also means their heights are different:
Specification Sidewall Radius Diameter Circumference Revs/km Difference
225/40-19 90mm 331mm 663mm 2082mm 480 0.0%
225/45-19 101mm 343mm 685mm 2152mm 465 3.4%
By you saying that you're getting the same widths back and front, you would have to get a taller series tire for the rear (like a 40 series in front and a 45 series in rear, you'd need to figure out what size 17's get you back to the numbers above), or else the car would be pretty useless on the highway with traction control on. The car doesn't care if you have different widths, just different heights. It might be a bit more tail happy though with equal widths in the dry, since you're reducing the effective grip of the rear, but for snow, thinner tires are generally better so they can bite down through the layers.