Because you get more pizza to eat, and if you don't finish it, you'll have breakfast tomorrow. Other than that fine reason, well, it's geometrically the better deal. Planet Money explains with an interactive that shows the price per square inch for 3,678 pizza places across the United States, based on data from Grubhub.
The math of why bigger pizzas are such a good deal is simple: A pizza is a circle, and the area of a circle increases with the square of the radius.
More pizza more problems
So, for example, a 16-inch pizza is actually four times as big as an 8-inch pizza.
And when you look at thousands of pizza prices from around the U.S., you see that you almost always get a much, much better deal when you buy a bigger pizza.
You get more pizza, and the business gets more money with minimal extra pizza-making effort. Win-win. Although, keep going on the horizontal axis and I bet that curve starts to curl up. Where can I get a ten-foot pizza?