The volume of a pizza doesn't scale linearly with its size. Instead, a pizza's volume is proportional to the square of the radius.

This means that a larger pizza not only gives you more pizza but a disproportionately larger amount.
A 20" pizza is not 2 but 4 times larger than a 10" pizza.

Furthermore, the relationship between a pizza's volume and it's price is not likely to be in alignment.

It is very difficult to perceive this difference, but in reality it is more than likely that you are paying a premium when ordering a smaller pie.

Take a look at some examples

Dominos

A 10in pizza has a volume of 78.5in³ and costs $7.85.
That's $0.10 per in³.

A 12in pizza has a volume of 113.1in³ and costs $9.95.
That's $0.09 per in³.

A 14in pizza has a volume of 153.9in³ and costs $12.6.
That's $0.08 per in³.

A 16in pizza has a volume of 201.1in³ and costs $14.7.
That's $0.07 per in³.

Pizza per dollar

10

12

14

16

Santa Lucia

A 10in pizza has a volume of 78.5in³ and costs $20.4.
That's $0.26 per in³.

A 13in pizza has a volume of 132.7in³ and costs $29.2.
That's $0.22 per in³.

A 15in pizza has a volume of 176.7in³ and costs $36.6.
That's $0.21 per in³.

A 18in pizza has a volume of 254.5in³ and costs $47.35.
That's $0.19 per in³.

A 30in pizza has a volume of 706.9in³ and costs $112.95.
That's $0.16 per in³.

Pizza per dollar

10

13

15

18

30

Compare other pizzas

Pizza 1

size (in)price ($)

Pizza 2

size (in)price ($)

A 10in pizza has a volume of 78.5in³ and costs $7.85.
That's $0.10 per in³.

A 12in pizza has a volume of 113.1in³ and costs $9.95.
That's $0.09 per in³.