I just stumbled across this article called 74,476 Reasons You Should Always Get The Bigger Pizza, written and posted by Quoctrung Bui.

This clever article assesses and confirms a suspician I've kept in the back of my mind for many years.

Buying the bigger pizza is an
exponentially greater deal.

Because of the properties of a circle, every increase in the diameter of said circle causes the area of that circle to increase at an expontial rate, compared to an equivelant increase in diameter of a smaller circle. In other words, going from a 6 inch pizza to a 7 inch pizza will obviously get you more food, but going from a 7 inch to an 8 inch will give your an even greater increase in quantity than your got from the 6 inch to 7 inch, with only a slight increase in price.

Check out this graphic (which is interactive if you click through to the original article), it explains a bit:

Quoctrung does a much better job of explaining, as does his interactive graph, so click through using the source link below or the image linked above to explore more.