The assumption that buying a bigger pack lowers the unit price is widely held and frequently wrong. Size pricing follows demand patterns rather than arithmetic.

Size choice reveals the buyer

Someone reaching for the large pack has usually decided they use enough of the product to justify it, which indicates steady demand and low sensitivity to the unit price.

Someone choosing the small pack is often trying the product, has limited storage, or wants a low outlay. That group is more responsive to the price on the shelf.

Pricing the small pack keenly and the large one less so follows directly from this, because the discount is placed where it changes a decision.

Costs do fall with size, but not proportionally

Packaging, handling and shelf space per unit all decline as pack size rises, so there is genuine cost saving underlying the traditional expectation.

Those savings are real but modest, and they are only part of the price. Where the retailer chooses to place margin can easily exceed the cost difference in either direction.

A large pack that occupies disproportionate shelf space or sells slowly can also cost more to stock per unit sold, reversing the expected relationship.

Promotions distort the comparison temporarily

Most reversals of the expected pattern are created by a promotion on the smaller size, which pushes its unit price below the larger pack for the duration.

Because promotions rotate, the size that offers the best unit price changes from week to week, and a habit formed during one cycle becomes wrong in the next.

The comparison therefore has to be made at the shelf on each occasion rather than settled once. Nothing about the pack itself makes the answer stable.

Unit pricing exists to make this visible

Shelf labels showing price per standard unit exist precisely because the comparison is otherwise difficult, involving division across inconsistent quantities.

The labels are only useful where the unit is the same across the compared items, and pack formats are sometimes chosen so that they are not directly comparable.

Where the units do match, the label removes the entire problem in a glance, which is why it is a more reliable habit than any assumption about size.

Quantity is not the same as value

A lower unit price on a larger quantity only converts into a saving if the whole quantity is used, which returns the decision to consumption rather than price.

For perishables and for products used occasionally, the smaller pack at a higher unit price frequently produces the lower total cost.

The useful comparison is therefore cost per unit actually used, which no shelf label can display and which depends entirely on the household doing the buying.