The nicest riding vehicle I have ever been in was a 2007 Yukon Denali. It had 20" wheels. The way the tire size works is this:
The 275/65 r20 wheels have a width of 275 mm The height of the tire from top of the rim to top of the tread is 65% of the width so 178.75 mm or 7.04" this means the tire has an overall height of 34.1"
The 275/70 R18 works out to 33.2" overall height with a profile height of 7.58"
I find my 2011 F350 SRW to be very stable while pulling and I wouldn't be surprised to learn at least part of that is coming from the 20" wheels.