It's makes perfect sense, and jeez Anthony I made the 'mistake' of going larger than 18s didn't I?
If you want to maintain the same section/sidewall height as an 18" tire on an SN-95 with its stock overall diameter, then you have to go up to a 19" tire on an S-197 with it's larger-than-SN95 overall diameter.
You still make no sense here. Go to tirerack.com and pick a set of 17's for your car, then check the overall diameter of the tire that is the largest recommended size. Then pick a set of 20's and check the diameter again. They will be almost exactly the same. What I'm saying here is that a 17 will have more sidewall, a 20 will have less, but they will be the same basic height. That's not to say that you might be able to search around and maybe find a 20 with a larger diameter...maybe.
Example: A 17"x9 wheel on an '05 Mustang has a recommended tire size of 235/55/17 which has an overall diameter of 27.177"
A 20"x9" wheel on an '05 Mustang has a recommended tire size of 255/35/20 which has an overall diameter of 27.027" which is within about 1/10th of an inch.