Good news! Mathematics has confirmed what we already suspected: You should most definitely be ordering a larger pizza.
But "I'm not hungry for that much pizza", you say? Allow me to refute your argument with two simple words: Cold. Pizza. But if you're still not convinced, check out this graph that Quoctrung Bui over at NPR made by crunching the pizza price data from over 74,000 pizzas from 3,678 different pizza joints:
There's also a handy interactive feature (which you can check out here) where you can see how many extra pizzas you would have to order to make up the total square inches that you would get at a larger size, and how much extra it would cost you.
For instance, here's what happens when you look at a 16-inch pizza, and how the smaller sizes stack up:
Less pizza for more money? NOT ON MY WATCH, AMERICA. I'll take the 16-inch pizza, please.