No you should be keeping the diameter of the combined rim and tyre the same otherwise your speedo/odometer will be out.
However a 18" rim is normally heavier than a 17" one and because it is a rotating mass the extra weight is about 4 times greater than just a static mass.
e.g. if your new wheels/tyres weigh an extra 4Kg each that is 16Kg for all 4 wheels and it has the effect of a 64Kg weight in the car.