One 18-inch pizza is more pizza than two 12-inch pizzas.
Let’s do some math. If a pizza has a diameter of 18 inches, that means it has an area of 254.47 square inches. And if a pizza has a diameter of 12 inches, then it has an area of 113.1 square inches. If you get two of them, you’re at 226.2 square inches, which is still less than one 18-inch pizza.