Not all costs are tied to size. Delivery expense for all sizes is exactly the same. Labor cost also vary somewhat from a direct correlation, same for the cost of running the oven, lights, insurance, etc etc. The short version is that large pies are more profitable than small ones because of the larger average transaction size. To the extent that 14" pies take away from 16" sales you will lose money, for every 14" sale that is an “upgrade” from a 12" pie you will make money.
On our menu a 14" pie is $2 cheaper for the pie and 35 cents less per topping than a 16" and the same amounts higher compared to a 12". The net effect is that the typical difference between a 16" and a 14" is about $3 with a 3 topping pie.