What It Means That Customers Aren't Very Good At Judging Product Sizes

So, supersizing is not necessarily an unalloyed benefit to the seller. JL

Pierre Chandon reports in Harvard Business Review:

One interesting consequence of this cognitive failing is that marketers who increase the size of food portions, houses, or even diamonds, are probably leaving money on the table — because customers don’t realize how much more they’re getting for their money.
People aren’t very good at judging changes in size. For example, I’ve found in my research with colleagues that when you make something bigger, people routinely underestimate how much bigger it is. This happens because, when estimating the volume of objects, we visually add instead of multiplying the changes in height, width, and length. So although increasing the height, width, and length of any object by 26% is enough to double its volume (because (1.26)³=2), to our eyes it will only look around 78% bigger (or 26%x3).
One interesting consequence of this cognitive failing is that marketers who increase the size of food portions, houses, or even diamonds, are probably leaving money on the table — because customers don’t realize how much more they’re getting for their money.
This isn’t going to change anytime soon; it is very difficult to improve people’s ability to estimate changing sizes. But the bias can work in your favor if instead of adopting a strategy of increasing unit sizes you reduce them — which, as I’ve argued elsewhere, can in many cases be a more successful packaging strategy than increasing product sizes. Just as people don’t recognize how much extra you may be giving them, equally they won’t recognize quite how much you might be taking away if your strategy is to reduce product size.The trick, as I found in a study with Nailya Ordabayeva, is to avoid cutting down on a single dimension of the product. When we presented study participants with two boxes, one 24% shorter than the other, we found that their estimates of the change in volume were reasonably accurate. We then presented them two boxes, one of which had twice the height of the other but less than half its depth and breadth, most people thought that there was virtually no difference in volume between them. In reality, however, the elongated box was 24% smaller in volume terms. This finding perhaps explains why many people believe that the 25 cl Red Bull slim cans look almost as big as the regular 33 cl cans and we drew on it to help a packaged good manufacturer downsize its product by 38% without losing perceived value compared to its competitors.

Attitudes to size are also very much influenced by comparisons — in buying takeout coffee, for example, people tend to prefer the size in the middle. So if you want to get people to switch to a smaller sizes with a higher margin for you, you would be advised to introduce a still smaller product so that your target sizes are in the middle of a range.
The actual labels matter as well, and the evidence suggests that trumpeting portion size can be counter-productive. In one experiment we offered consumers three increasing portions of fries. We found that labeling them “Mini,” “Small,” and “Regular” rather than “Regular,” “Big,” and “King”  increased consumption intentions by 23% because some people wanted a regular size no matter how many fries it really contained.
Similarly, after a point, there’s often an inverse relationship between pleasure and the amount of food eaten. In one of a series of recent studies with Yann Cornil, we asked one group of people in a broad sample of participating consumers to remember the sensory pleasure (in terms of taste, smell, and texture) that they experienced when eating foods like chocolate mousse or vanilla ice cream.  The other people we surveyed were not asked these questions. We then asked everyone in the experiment to choose between smaller or larger portions of tasty foods. What we found is that people that we had previously asked to recall the sensory pleasure they took in the foods systematically opted for smaller portions and were willing to pay more for them.

As these examples show, once marketers start thinking strategically about package or portion sizes, they can find ways to get customers to be happier with less. A win-win for business, sustainability, and consumer experience.