The medium can make the large look like a bargain
A strangely priced drink size may be there to shape how the other options look. If the medium costs nearly as much as the large, the extra cost of upgrading seems small. A customer who might have chosen the small can start thinking, “For just a little more, I get the large.” The medium acts as a comparison point that nudges the decision upward.
In marketing, this is called the decoy effect: adding an option that is less appealing than another can make that other option seem like the obvious choice. The medium need not be completely useless. It may simply offer less value per ounce than the large, making the larger size stand out.
Why price the sizes that way?
A shop has reasons to care about more than the number of drinks sold. It wants customers to choose options that cover costs and contribute to profit. The ingredients in a larger drink usually cost more, but not necessarily in proportion to its price: the cup, lid, and much of the work involved are similar across sizes. That can make an upgrade attractive to the shop as well as to the customer.
The menu also helps customers compare. With only a small and a large, the large may seem expensive. Add a medium priced close to it, and the large can feel like the better deal. This does not mean every unusual price is a deliberate trick; prices can also reflect costs, local competition, or how a shop expects its customers to order.
The choice still depends on what you value
The comparison works only if the customer cares about getting more drink for the money. Someone who wants less caffeine, has a set budget, or dislikes wasting a half-finished drink may still prefer the small—or the medium. The useful question is not just which size looks like the best deal, but whether the extra drink is worth its extra cost to you.
