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.

So next time you're figuring out what size pizza to buy, use the price per square inch, not just what feels right.