Domino's is hardly considered pizza by most but it's $7 for a 12in. A 18in is $20. That's almost 3 pizzas. And the 12in has 2 toppings. The 18in has 0 toppings.
it is not about a difference in value, but how they harmonize together. the interior of the pizza gets its strength from the crust, and vice versa, but these elements are only truly in harmony once the pizza achieves an ideal pie/crust ratio.
Dominos actually got better. It's not amazing but they took it on the chin a few years back and were like, "our pizza sucks. We need to do better" and they actually improved it quite a bit.
The math only really works for 18+ inch pizzas though. The pizza places around me don't even offer 18 inch pizzas. 14" large or 16" XL are the highest they go. In that case at most places near me, two twelves is often cheaper per square inch and does have more area than one 14" or 16". Especially since Domino's usually has coupons for two 12s that make it significantly cheaper than 1 L or XL.
Factor in the crust ratio of those though. We're talking 1.5 inch of crust, so 16" vs 12" is actually more like comparing 13" to 9" of pizza with cheese and topping. 132 v 64 square inches. You're getting 70 squares inches of crust on that 16", and 49 square inches of crust on the 12 inch. So more total food on 2 12s, but a lot more crust than one 16.