Shoppers instantly know when products are shrunk
When Toblerone reduced the size of their chocolate bars it faced a revolt by disgruntled shoppers.
Now psychologists have found sharp eyes shoppers can accurately assess from a glance when they were getting less for their buck.
They are more likely to complain or boycott products that have been downsized as when Toblerone increased the gap between its triangular chunks.
Yet they are also less adept at spotting when they are supersized so don't realise how large the portions have become and eat more.
This has implications for firms who try to reduce portion size to help tackle the obesity epidemic.
For instance Coca Cola was once sold in 6.5 fluid ounces or 192ml bottles but now in fast food outlets a regular drink was 32 oz or just under a litre.
Professor of marketing Pierre Chandon at the business school INSEAD said: "Our brain is very bad at judging quantity increases, but surprisingly accurate at judging quantity decreases
"Supersizing food portions is a lose-lose proposition: Consumers don't realise how much food is available, they refuse to pay a fair price for it, and end up eating more than realise.
"Companies should consider downsizing back to what used to be a regular portion size not so long ago.
"But they need to downsize smartly, leveraging what we know about size perceptions, otherwise consumers will reject it".
The study published in the Journal of Experimental Psychology: Genera involved five studies and 4,842 size judgments.
Results showed people, including experts such as professional chefs estimate quantity decreases more accurately than quantity increases.
On average, they found that a portion that is doubled in size is judged to be only 72 per cent larger than the original size, a strong underestimation.
On the other hand one that is halved appears to be 53 per cent of the original size, which is a very good approximation.
In one "supersize" experiment, 510 participants were asked to take a look at five different portions of M&Ms in plastic cups.
The cups had 37, 74, 148, 296, and 592 sweets respectively.
They were how many were in the smallest portion and then asked to estimate the number of M&Ms in the other four portions.
The average estimates were 57, 102, 184, and 296 so underestimating by half the largest portion.
But in a reverse "downsizing" experiment when told the number of sweets in the largest cup and asked to estimate the numbers in the other four their average estimate was 346, 163, 74, and 36.
They only missed the size of the smallest cup by one sweet.
The researchers hypothesised this asymmetry exists because there is a natural lower bound or a zero point when portion sizes decrease.
In other words, a decreasing portion cannot go below zero.
When portions increase, however, they can theoretically grow to infinity and without an upper bound, it is hard for people to estimate how big something has become.
To test this they told participants the upper limit of the largest container was 629 chocolate sweets.
In the supersizing experiment they judged the largest container to hold 528 sweets much closer to the actual numbers.
When told the upper bound judgments of size increases were no longer less accurate than judgments of size decreases.
Assistant professor Nailya Ordabayeva at Boston College said: "Our study suggests a number of strategies that can improve consumer decisions in the face of quantity increases vs. decreases
"This improved visual accuracy, in effect, makes people less averse to, and more receptive towards, healthier downsized portions and packages."