The American trend for super-size portions has come to the UK and is contributing to rising levels of obesity, health experts warn.
Hamburgers are 112% bigger than they were 20 years ago
They say being seriously overweight can lead to cancer or heart disease.
The World Cancer Research Fund (WCRF) says 'two-for-one' or 'meal deal' offers encourage people to eat too much.
It calls on patients to watch what they eat - and companies to make it easier to choose to eat healthily.
HOW TO DEAL WITH LARGE PORTIONS
If there is a choice, opt for the smaller size
Do not feel you have to clear the plate
Ask if you can take leftovers home
Only eat what you need
Be aware of bigger pack sizes and if you do buy special offers, do not eat it all at once
But take advantage of special offers on fruit and vegetables
Share large portions of sweets of popcorn
One in five people are overweight or obese, the charity warns.
It says the problem is being exacerbated by "portion distortion", widespread in the US, where enormous meals are served as a matter of course.
It means Americans under-estimate the amount they eat each day and then indulge in "unconscious eating".
In 1984, 46% of Americans were overweight and 14% obese. By 2000, the numbers had increased to 64.5% and 30.5%.
Over the last 20 years the charity says food portions have got ever bigger. Hamburgers are 112% larger than in 1982, pasta servings 480% larger, and chocolate chip cookies 700% bigger.
Dr Jeffrey Prince of the WCRIF said: "Between 1980 and the present, portion sizes ballooned, and so did people. These two trends occurred simultaneously. Common sense tells you there must be a connection."
But the charity says the food industry denies there is a link between bigger food portions are directly linked to obesity.
Doctors say consumers must realise that, though special offers can be appealing, they are bad for their health.
They warn that obesity increases the risk of developing Type 2 diabetes, heart disease and breast, colon, endometrial, oesophageal and kidney cancers.
The World Cancer Research Fund has missed its target
Martin Paterson, Food and Drinks Federation
Professor Philip James, chairman of the International Obesity Task Force, speaking on behalf of the WCRF, said: "This is a warning that needs to be heard.
"People seem to be aware that enjoying a diet of fruit and vegetables is important, although few actually act on this, but seem far less aware of the real need to reduce sugar and fat in their diets to help maintain a healthy body weight."
He added: "We must recognise that the food and drinks industry, in offering consumers ever larger portion sizes, may be contributing to the unhealthy nutritional environment which promotes overweight and obesity.
"The industry can make it easier to make healthy choices and not over-promote the 'super-sizing' marketing model of the US.
Ms Paula Hunt, a State Registered Dietician, who is working with the WCRFI, said: "People think they're getting a bargain when they get more food for just a few pence more.
"What isn't a bargain are the extra calories and fat that comes with these 'value meals'.
"But, by exerting a bit more control and deciding for ourselves how much we should eat, we can face bigger portions without fear and stay at a healthy weight.
Limit weight gain in adulthood to no more than 11 pounds
Do one hour of moderate exercise each day and one hour a week of vigorous activity
Eat plenty of cancer-fighting vegetables, fruits, whole grains and beans
But do not eat more food than you need - even if it is healthy
Lose weight carefully
"Most weight loss success stories centre around reduced portion sizes. It's a simple fact, if you eat less, you'll lose excess weight."
Richard Elworthy, of the charity CancerBACUP, said: "It's the type of food as well as portion sizes that's causing the problem.
"We could actually reduce our risk of developing illnesses such as cancer if we increased our intake of fibre-rich fruit and vegetables."
But Martin Paterson, Deputy Director General of the Food and Drinks Federation, said: "The World Cancer Research Fund has missed its target.
"UK food and drink manufacturers provide many of our favourite products in a wide variety of sizes and styles to suit consumers' varied nutritional needs and tastes.
"This widens people's choices for building a healthy, balanced diet."
But Tim Lang, professor of food policy at City University in London, said the industry was showing "undue defensiveness".
"The British are eating more and exercising less. This is silly of us. We have to reverse that trend.
"How much better it would be to keep consumers alive and healthy longer."