A familiar product can cost the same as it did a year earlier while containing less. The change is deliberate, disclosed on the label, and largely invisible in practice.
Price memory is stronger than quantity memory
Shoppers reliably notice when a price rises, because price is the number they interact with at the shelf and at checkout.
Net weight is printed but rarely read. Few people could state the contents of a cereal box or a bag of chips they buy weekly, which makes quantity the softer variable to adjust.
Manufacturers facing rising input costs therefore have two routes to the same margin, and the one that does not touch the price point attracts less resistance.
Packaging is redesigned to conceal the change
Reductions are usually paired with a new package, because a smaller quantity in the old container would be obvious from the fill level or the shelf footprint.
Depth, base indentations and wall thickness change more often than height and width, since the dimensions a shopper sees on the shelf stay constant.
New graphics frequently accompany the change, and claims such as a new look or improved recipe give the redesign a reason unrelated to the quantity.
Price points are sticky for structural reasons
Many categories are anchored to familiar price points that have persisted for years, and crossing one produces a disproportionate fall in sales.
Vending, convenience and promotional formats reinforce this, since machines, multibuy structures and advertised offers are all built around specific figures.
Holding the price point while reducing contents preserves all of that infrastructure, which is a practical reason the approach recurs across many categories.
Retailers are not neutral parties
A grocer negotiating with a supplier over a cost increase may prefer a size change to a price change, because shelf prices affect the store's own price perception.
Private label ranges follow the same logic, and store brands change sizes for the same reasons national brands do.
Some grocers have made a marketing position of flagging reductions on shelf tags, which works precisely because the information is otherwise so easy to miss.
Unit price is the only stable comparison
Because quantity is the moving part, comparing package prices across time or across brands measures the wrong thing.
Price per ounce or per unit strips the packaging out of the comparison, and it is the figure that reveals a reduction immediately even when the headline price has not moved.
Unit price display is required by regulation in a number of US states and provided voluntarily by many chains elsewhere, so the figure is usually available on the shelf tag.