A 9 inch pizza is a one-person pizza. It is the size used in Deal 6, where you get any 9 inch pizza with fries and a drink for £5.
A 12 inch pizza suits two lighter appetites, or one person who wants leftovers. Every multi-pizza deal - Deals 1, 2, 3 and 4 - uses 12 inch.
For a group, three 12 inch pizzas on Deal 1 at £18 works out cheaper per slice than ordering 9 inch pizzas individually.
Toppings are identical across both sizes, so the only decision is how much you want to eat.