Because math really does come in handy. From a guy who knows pies and pi, simple geometry reveals that two 8-inch pizzas are not equivalent to one 12-incher:
... the waitress came by with an 8-inch round pizza, explaining that another waitress had mistakenly given our pizza to someone else. She said we could have this 8-inch pizza now, and she’d have the cook throw another 8-inch pizza in the oven for us. She claimed that we’d be getting more total pieces of pizza, so this was a good deal for us.
After doing some quick mental math (area of a circle = pi*radius². Two 8-inch pizzas = 2*pi*(4)² = 32*pi square inches, One 12-inch pizza = pi*(6)² = 36*pi square inches), I told her we’d be missing out on over 12 square inches of pizza, so we’d rather just have the one 12-inch pizza. She complied, and as a nice bonus (probably because she was impressed by my quick geometry skills), she let us have the extra 8-inch pizza anyway. Score one for geometry!