You are correct hdave. Has nothing to do with wheel size. Depending on the wheels the 18s might actually weight less. XXRs are usually pretty lightweight usually they have the wheel weight with the wheel size, offset, lug pattern
I noticed an MPG loss simply by adding wider tires to stock rims. I went with 225 instead of 215. I got Nitto Motivo's which I do not believe are low rolling resistance. I noticed a noticeable drop of 1-2 mpg.