A round plate weighs more than a square plate of the same size because it has a larger surface area. To calculate the weight of a round plate, you need to multiply the weight of a square plate of the same size by the surface area of the round plate. For example, if a square plate weighs 1 pound and has a surface area of 8 square inches, a round plate with a surface area of 16 square inches will weigh 4 pounds.