More pizza is always a good thing, unless it's the rare occurrence of bad pizza. When the pizza is good, more is better. Believe it or not there is math that supports the notion we should all be eating more pizza. The folks at ASAP Science have posted a video explaining from a mathematical standpoint why it is always better to order larger pizzas.
Summary/TLDR; A sixteen inch pizza has four times as much pizza as an eight inch pizza, but usually only twice as much (or less).
The video above shows us that it if you look at the amount of pizza you get in proportion to the price, it's more economical to buy a larger pizza. The amount you get can be determined by looking at the area of the pie. Side note, technically you could look at the volume, but the thickness of a small vs medium vs large pizza will be the same. The area of a circle is determined by the formula:
An eight inch pizza will have a radius of four inches, and plugging this into the equation gives you roughly an area of fifty inches. A sixteen inch pizza will have a radius of eight inches, which gives you an area of two hundred inches, which is four times as much pizza as the eight inch, but you'll never find an eight inch pizza that costs four times less than a sixteen inch pizza. The video goes on to illustrate the price per unit difference between a small, medium, and large pizza to further drive home the point.
Another tangential consideration is the ratio of crust to pizza. As you go larger with pizza, the ratio of crust to pizza decreases. This means you have more pizza to go along with your crust as you get a larger pizza, and less with a smaller pizza.
So now as if you needed a reason to get a large pizza, it can be said that you save money by getting a large or extra large.