You should always order the bigger pizza. While an 8“ pizza is roughly 50 square inches, a 16” pizza is 200 square inches, which is 4 times bigger but most pizza places charge by radius and don’t consider the overall area of the pizza. An NPR study found that one 20” pizza is actually bigger than two 14” pizzas, but is also, on average, $9.00 cheaper. Source
Okay, but this is actually really useful for university students. Every cent counts, and if you have a minifridge (hell, even if you don’t, wrap it up tightly in foil while it’s still warm and you can just keep it out at room temp for a couple days and it’ll be fine) the inevitable leftovers won’t be a problem.