Is the bigger pack actually cheaper? We checked 3,829 of them
"Buy the big one" is good advice about eight times in nine. Here is what happens the ninth time.
Buying the bigger pack is the oldest supermarket rule there is, and it mostly works. But it is a rule of thumb, not a law — and when it fails it tends to fail on the exact aisle you were being sensible in. So we measured it: 3,829 pairs of the same product in two different sizes, at the same retailer, across the Australian supermarkets we track in September 2026.
The rule holds — about eight times in nine
Taking every product we could match to a larger version of itself at the same shop:
- 87% of the time the bigger pack really was cheaper per 100g, per litre or per item.
- 11% of the time it was dearer per unit — about one pair in nine.
- The rest were line ball.
So the instinct is sound. It is the one-in-nine that costs you, because that is precisely when you are not checking.
Half the failures are a special in disguise
Of the pairs where going bigger cost more per unit, just over half had the smaller size on special that week. Nothing is wrong with the shelf price of the big one; the small one is simply discounted, and a discount on the small size beats bulk economics surprisingly often.
That has a useful corollary: the failure is temporary. The same two packs may well go back to the normal order next week, which is why a habit beats a memory here — check the unit price on the day rather than learning a rule about a product.
A smaller group — around one in eight of the failures — had the bigger pack on special and it was still dearer per unit. A yellow ticket is not, by itself, evidence of value.
The other half are just priced that way
The remainder carry a higher per-unit price on the big size with no promotion involved at all. In our September 2026 snapshot the gap was worth having an opinion about: the median penalty for going bigger, in the cases where bigger lost, was about 19%.
The pattern turns up in unglamorous places — pantry staples in 1kg versus 2kg, dry pet food in 3kg versus 8kg, toothpaste in two tube sizes, salad leaves in two punnet sizes. These are not exotic products. They are the weekly shop.
The habit that fixes it
Large Australian supermarkets are required to show a unit price — the "per 100g" or "per litre" figure — next to the selling price, precisely so two sizes can be compared honestly. It is usually the smallest text on the ticket, and it is the only number on it that answers the question you are actually asking.
Mai Pantry puts that figure on every search result, next to the pack size, for every retailer at once — so the 250g and the 375g sit side by side with their real cost already worked out.
Check the small number, not the big one.