That is according to Aradhna Krishna, marketing professor at the University of Michigan, who identified a link between eating habits and food labels.
She gave research participants two types of cookie sizes to choose between and sat back to see how they behaved when it came to 'medium' and 'large' labelled biscuits.
What she found was that people tended to eat more of the cookies described as medium, despite the fact they were actually as big as the ones entitled large.
As such, people were more influenced by the portion size they believed they were chowing down on than the quantity they were actually eating.
"Just because there's a different size label attached to the same actual quantity of food, people eat more," professor Krishna said.
"But also, think they've not eaten as much."
This has far reaching consequences considering some fast food eateries serve the same portion of fries, with one labelling them small and another describing them as medium.
Professor Krishna drew attention to the fact that food portions and drink sizes have escalated over the decades so that the quantities we are consuming these days are far bigger than our grandparents, for instance, would have enjoyed.
She is calling for a more standardised approach to food labelling so that restaurants offering different sized portions all provide the same amount under each label.
"All I'm saying is that sizes should be made more uniform, and that will only help the consumer because you'll know what you're getting," she said.
New York City recently banned the sale of sugary drinks of more than 16 ounces, with the UK government considering a similar move.
This is in a bid to fight obesity, which currently affects 26 per cent of adults in England.