On a café menu, a $3 small and $5 large coffee make a $4 medium feel sensible.
That menu feeling can be designed. Dan Ariely called it the decoy effect: a third price makes one existing choice look better.
A $9 large popcorn beside a $7 medium can make the medium look like a bargain.
The reason is comparison: the large gives the medium a target, while the small loses attention. Without the popcorn, customers compare prices directly. The menu changes the question from “What do I need?” to “Which deal looks least foolish?”