Usually yes — a single 18-inch pizza has about 254 square inches of pizza, while two 12-inch ones have only about 226.
Area grows with the radius squared, so doubling the diameter quadruples the area. An 18-inch pizza covers 2.25 times the area of a 12-inch one. That means you often get more pizza per dollar with one large than two smalls. Geometry defeats intuition at the pizza counter.
