They are usually lighter for the same overall diameter. Helps with mpgs.
Actually the wheel + tire assemblies are heavier for a given overall diameter - metal is a lot heavier than rubber filled with air. You might get a small MPG benefit due to increased stiffness, but the trade is:
1. Stiffer Ride (observed LT275/70R18 vs the 20in OE on same 2500Ltd)
2. More likely to damage expensive rims on curbs (been there, done that)
3. Poorer handling due to higher unsprung weight (trivial on a truck)
4. More expensive (how is less material in a tire more expensive - because they can claim lower volume and there is limited competition)
5. Harder to find / $pecial Order / Less options
6. Frequently lower load capacity at a given width and diameter
On the plus side, on a race car, you can put bigger brakes under larger diameter wheels which more than offsets the unsprung weight issue and increased stiffness there is frequently a benefit.
So it gives vehicle a sexy racing look and manufacturer can add $1200/2000 price tag to sticker for what costs them about $400 extra at the OE level.