The "squared" in Pi(R)^2 means that the area of a pizza grows
exponentially polynomially in relation to its diameter. As an interactive graph on Planet Money demonstrates, pizza places generally underprice their bigger pies relative to the amount of food contained in each. This is probably because energy and labor inputs account for the largest slice of the pizza-baking ahem pie, and ingredients are way down on the balance-sheet. Whatever the reason, if you're interested in getting more food for less money, larger pies are almost always a substantially better deal.
74,476 Reasons You Should Always Get The Bigger Pizza [Quoctrung Bui/Planet Money]