My 2015.5 2500HD GMC denali has 20inch rims, tires have the same or higher load rating as the 18" rims or 17" rim tires.
Advantages I've seen vs. my 2004 is the 20" with lower profile make the truck more responsive and less wander. Downside is there aren't as many choices for tires when it's time to replace them.
and BTW the tire diameter is exactly the same for 20's, 18's and 17's. In fact chevy uses a 18" as the spare regardless of if you have 17 ,18 or 20's as the factory rim.
And why did they bigger?? For one they increased the brake rotor diameter, larger brakes.