To begin, they have had the audacity to make pizzas in sizes in less than the scientifically proven optimal size. However, they add insult to injury by calling these smaller sizes "large". Since pizza size increases as the square of half the diameter, decreasing the diameter doesn't have an intuitive effect on the actual reduction in pizza.
To help illustrate the atrocity being done here, I've compiled this table of various pizza sizes. I've included the name Pizza Hut (or the others) call them, as well as their name to rational humans. Next, is the diameter and the area. The slices equivalent lets you know how many slices of a normal 16" pizza you would be getting at the other sizes; they likely are all cut into 8 slices. % of a real pizza is exactly what it sounds like.
|Pizza Hut Name||Actual Name||Diameter (in)||Area (in^2)||Slices Equivalent||% of a Real Pizza|
I've made a calculator to help compare prices across pizza sizes.