This seems backwards to me. 8 + 8 = 16, which is more than 12. How can one 12-inch pizza have more pizza than two 8-inch ones?
Yes, the single 12-inch pizza wins. Its radius is 6 inches, so its area is 36π, about 113 square inches, while each 8-inch pizza has radius 4 and area 16π, so two of them make 32π, about 101 square inches. Area grows with the square of the radius, which is why a slightly bigger pizza is a lot more food.
Accepted answer
The trick is that pizza size depends on area, not diameter, and area grows with the square of the radius: A = πr². A 12-inch pizza has a 6-inch radius, so its area is π × 6² ≈ 113 square inches. Two 8-inch pizzas each have a 4-inch radius, so each is π × 4² ≈ 50.3 square inches, or about 100.5 together. The 12-inch pizza wins!
Adding diameters (8+8=16) treats pizza like a 1D length. Pizza is 2D, so you have to square the radius, not just add diameters.