Why the Big Box Is Not the Cheap One
Bigger is usually cheaper per unit, and usually is not a rule. The quantity surcharge is mostly an artefact of promotions, which is why it keeps happening without anybody deciding to mislead you.
Key takeaways
- Quantity discounts are genuinely common, which is why the heuristic that bigger is cheaper exists and mostly works.
- A quantity surcharge is when the larger size costs more per unit, and it is usually created by a promotion on the smaller size.
- Clerides and Courty report that sales of the large size fall only slightly when this happens, despite the small size being the cheaper option.
- They read this as rational inattention: checking every unit price costs attention, and the heuristic is right often enough to keep.
- Chioveanu and Zhou show why it is not competed away, since firms randomise price frames to obfuscate comparison and more competitors can mean more complexity.
Two boxes of the same washing powder sit on the shelf. One is twice the size. You reach for the big one, because everybody knows the big one is cheaper per wash, and some of the time you are wrong. How often is the question, and the honest answer is that it depends on the category and on whether a promotion is running that week. Sofronis Clerides and Pascal Courty document the pattern in two datasets across four product categories, which is the best evidence there is and still not a general rate. If somebody quotes you one national percentage for how often big packs cost more per unit, ask which four categories it came from.
The arrangement
Quantity discounts are real and they are the norm. Selling more units in one transaction lowers packaging, handling and shelf costs, and firms share some of that back. Mark Armstrong's review of nonlinear pricing works through the conditions under which offering them is profitable, under monopoly and under competition, and covers what changes when consumers have bounded rationality. The short version is that the conditions are common, which is why the intuition exists at all.
The exception has a name. A quantity surcharge is when the larger size costs more per unit than the smaller one, and it is common enough in the categories anyone has looked at to be worth checking for.
The mechanism
The usual cause is a promotion, not a conspiracy about box sizes.
Clerides and Courty, writing in the Review of Economics and Statistics, describe the pattern: a firm markets a product in two sizes and runs a promotion on the small one. For as long as that promotion lasts, the large size costs more per unit than the small, and the shelf now contains an arbitrage opportunity sitting in plain sight.
Their finding is what happens next. Sales of the large size decrease only slightly, despite the small size being the cheaper option. Shoppers keep buying the big box while the discount is running.
Clerides and Courty offer rational inattention as an interpretation consistent with that pattern: people economise on what they bother to work out, checking every unit price costs attention, and the heuristic is right often enough to keep. Worth saying that it is an interpretation and not a test. Pack loyalty, a preference for fewer shopping trips, storage, or simply not registering that a promotion was running would all produce the same purchase data, and none of them has been ruled out.
If the inattention reading is right, it is a more forgiving account than the one usually offered and also worse news, because a heuristic that is usually correct is one you will not notice failing.
The obstacle
Unit pricing exists to fix exactly this, which raises the obvious question of why the effect persists.
Unit pricing is required on shelf labels across the United Kingdom and the European Union, with the rules now diverging since Brexit and the detail varying by product type. Part of the answer is that the unit itself varies. Per 100g, per kilo, per wash, per 100ml, per sheet. Two products in the same category, next to each other, sometimes carry different denominators, and converting between them at the shelf is arithmetic that people are doing while holding a basket.
Part of it is typography. The [unit price](https://thewhyweproject.com/articles/why-do-mpg-figures-mislead-car-buyers) is routinely the smallest number in the display, set below the headline price, and the headline price is the one being shouted about. A car's miles per gallon raises a related problem, because the unit printed on the label changes how large an improvement looks.
And part of it is the promotion itself, because a large yellow label announcing a saving is an instruction about where to look.
Be precise about which half of this is solid. That shoppers keep buying the large size through a surcharge is scanner data on real purchases analysed by economists, and it does not have the fragility that has undone so much of the psychology this publication covers. Why they keep buying it is not established by that data at all, which is why the rational-inattention reading above is labelled an interpretation and why four alternatives remain open. The behaviour is well evidenced. The explanation is not, and we would rather leave it that way than borrow confidence from one for the other.
The comparison
There is a formal reason none of this gets competed away, which this publication has set out at length in its piece on delivery charges and will not repeat here. The short form: when firms compete on how prices are presented as well as on the prices themselves, obfuscation can be an equilibrium rather than a lapse, and adding competitors does not reliably fix it.
The shelf version is more concrete anyway. Two products, two denominators, one promotion label, and a shopper holding a basket.
The verdict
Our reading is that the quantity surcharge is mostly an artefact of promotions rather than a standing deception, and that this makes it more common rather than less.
Nobody needs to decide to mislead you. A promotion on the small size, run for ordinary commercial reasons, creates the surcharge automatically, and the shopper's reasonable heuristic does the rest. That is why it recurs in category after category without anybody being able to point to a culprit.
We would also resist the tidy advice that follows. Telling people to check unit prices on everything asks them to spend the exact resource the inattention reading identifies as scarce. The realistic version is narrower.
Check the unit price on the four or five things you buy every week and never again. Those are where the money is, the comparison only has to be made once, and it stays true until the packaging changes.
Be most suspicious where there is a promotion. That is the condition under which the surcharge appears, and a yellow label on the small size is the specific signal.
And treat a mismatch in units as information. When two neighbouring products are priced per kilo and per 100g, somebody chose that, and the choice was not made to help you.
The line to keep
Bigger is usually cheaper. Usually is a habit dressed as a rule, and it works often enough to survive being wrong.
Sources
Armstrong, M. (2016). 'Nonlinear pricing.' Annual Review of Economics, 8, 583-614. Examines when it is profitable to offer quantity discounts and bundle discounts, and the impact of consumers with bounded rationality. doi:10.1146/annurev-economics-080614-115650
Clerides, S., & Courty, P. (2017). 'Sales, quantity surcharge, and consumer inattention.' The Review of Economics and Statistics, 99(2), 357-370. doi:10.1162/rest_a_00562. Verified at the published record 2026-09-25; an earlier draft of this article cited the 2010 working-paper version. Quantity surcharges occur when a firm markets a product in two sizes and promotes the small one, so that the large size costs more per unit; sales of the large size decrease only slightly despite the small size being cheaper, which the authors describe as consistent with rational inattention. This publication cites it as a working paper and has not obtained a published version.
Chioveanu, I., & Zhou, J. (2013). 'Price competition with consumer confusion.' Management Science, 59(11), 2450-2469. doi:10.1287/mnsc.2013.1716
Yao, J., Oppewal, H., & Wang, D. (2020). 'Cheaper and smaller or more expensive and larger: how consumers respond to unit price increase tactics that simultaneously change product price and package size.' Journal of the Academy of Marketing Science, 48, 1075-1094. Cited by title; no abstract was retrievable and this publication has not read it. doi:10.1007/s11747-019-00716-z