Reducing the portion sizes offered in supermarkets, restaurants and at home would help reverse the obesity epidemic, say researchers.
They say their review of 61 studies provides the “most conclusive evidence to date” that portion size affects how much we unwittingly eat.
The team at the University of Cambridge also said smaller plates, glasses and cutlery helped people eat less.
Experts said people were “reluctant” to leave a plate with food on it.
Their data, published in the Cochrane Database of Systematic Reviews, shows that when people are offered more food they will eat it.
And the team warns that in recent decades portion sizes have been increasing.
How portion sizes have changed
On average between 1993 and 2013:
- Shepherd’s pie ready meals almost doubled in size
- Bagels increased in size from 70g to 86g
- A family pack of crisps increased 50% from 100g to 150g
- A portion of peanuts is now 80% larger
- An individual chicken pie is now 40% bigger
About two in three adults in the UK are either overweight or obese, which increases the risk of heart problems, type 2 diabetes and cancer.
The findings, which are based on 6,711 people taking part in a wide range of clinical trials, suggest that eliminating “large portions” could cut up to 279 calories a day out of people’s diets.
Dr Ian Shemilt, from Cambridge’s Behaviour and Health Research Unit, told the BBC News website: “This is the most conclusive evidence to date that people consistently consume more food and drink when given larger portions, packaging or tableware.
“Consumers do have a role to play – for example, all of us can reduce the size of plates or glasses we use and put pressure on the pubs and restaurants we visit by asking for a smaller portion.”
The team also says government measures to force smaller packs to offer better value for money and upper limits on the size of energy-dense foods would help people lose weight.
Dr Alison Tedstone, chief nutritionist at Public Health England, said: “This study clearly demonstrates that reducing portion sizes is a successful way to cut calories.
“It’s important to keep an eye on portion sizes when cooking, shopping and eating out to avoid overeating and help maintain a healthy weight.”
Prof Brian Ratcliffe, emeritus professor of nutrition at Robert Gordon University in Aberdeen, said: “People seem to be reluctant to leave or waste food and so consume what they are served or find larger portions more attractive.
“A limited number of restaurants and food outlets already offer more than one portion size with appropriate pricing differentials and this seems to be a way forward to help people to avoid overconsumption.”