Which sounds odd.
The difference in diameter is -0.21% in the rear and -0.44% in the front.
They said that because of the torque difference, I might risk damaging my car.
To me it sounds absurd, since the difference is minimal, but maybe in the grand scheme of things it proves to be a differential destroyer or something.
Anyway, since everything is expensive when it comes to servicing my car, they also said that it would cost 2000 dollars to get the set of 18in rims and the respective tires.