When costs rise, a producer can raise the price or reduce the quantity. The second option raises the unit price while leaving the figure on the shelf unchanged.

Price memory is stronger than size memory

Shoppers hold reasonably accurate recall of what familiar items cost, and an increase in that figure is noticed and resented.

Recall of pack contents is much weaker, particularly where the quantity is printed in small type and the pack shape is adjusted to disguise the change.

Reducing quantity therefore raises the effective price through the channel shoppers monitor least, which is the entire basis of the practice.

Price points constrain the alternative

Many categories sell at established price points, and moving above one produces a disproportionate fall in volume rather than a gradual one.

Where a product sits just below such a threshold, any increase crosses it, which makes a straightforward price rise unusually costly.

Holding the price and reducing the contents keeps the product on the correct side of the threshold, preserving both the price point and the margin.

Redesign provides the cover

Size reductions are typically accompanied by new packaging, a changed shape, or a claimed improvement, all of which give the shopper a reason for the pack looking different.

Thicker bases, deeper indentations and altered proportions maintain the apparent size of the pack while reducing the contents.

These changes cost money to implement, which is why the practice tends to arrive in waves during periods of sustained input cost pressure rather than continuously.

Unit pricing is the only reliable check

Comparing price per standard unit removes the effect entirely, since it measures exactly the quantity that changed.

This is why unit price labelling requirements exist and why their prominence on shelf edges has been a recurring subject of regulatory attention.

The label works only where the unit is comparable, so changes that also alter the format can still obscure the comparison.

The change is difficult to reverse

Once contents have been reduced, restoring them when costs fall requires either a price cut in real terms or a visible increase, neither of which is attractive.

Pack sizes in many categories have therefore ratcheted steadily downward over long periods without corresponding recovery.

The cumulative effect is a substantial rise in real price that never appeared as a price increase at any point in the sequence.