Scientists have long marvelled at France's low levels of obesity.
The French eat more fat but less calories
After all the French consume much more fat than Americans, downing mountains of cheese, croissants and pastries.
However, just 7% of French adults are obese - three times lower than in the United States.
Now, researchers on both sides of the Atlantic believe they have cracked the riddle. The answer, they say, is simply smaller portions.
While the French eat more fat, they consume fewer calories than their American counterparts.
Researchers from the University of Pennsylvania and CNRS in Paris compared the size of restaurant meals and even portions in cookbooks in France and the United States.
They weighed portions served up at 11 comparable outlets in Paris and Philadelphia. These included fast food outlets, pizzerias and ice cream parlours.
They found that Americans received substantially more on their plate.
While the average Parisian portion weighed 277 grams, the Philadelphia equivalent came in at 346 grams - 25% more.
The difference was even more stark when the researchers compared a number of well-known international chains with restaurants in Paris and Philadelphia. They found that meals in the US were consistently much larger.
In Chinese restaurants, Americans were given up to 72% more than they would have been given if they ordered the same thing in France.
The researchers also compared bars of chocolate, cans of soft drink and hot dogs on either side of the Atlantic.
Again, American portions were considerably larger. A bar of chocolate in Philadelphia was 41% bigger than the same product in Paris. A soft drink was 50% bigger and a hot dog was 63% larger.
Even yoghurts were bigger. The American yoghurt was 82% bigger than its Paris equivalent.
"While the French eat more fat than Americans, they probably eat slightly fewer calories, which when compounded over the years can amount to substantial differences in weight," said Paul Rozin, professor of psychology at the University of Pennsylvania.
He said smaller portions could help to tackle the rise in obesity in the US.
"Many studies have shown that if food is moderately palatable, people tend to consumer what is put in front of them and generally consume more when offered more food," said Mr Rozin.
"Much discussion about the 'obesity epidemic' in the US has focused on personal willpower, but our study shows that the environment also plays an important role and that people may be satisfied even if served less than they would normally eat."
The study is published in the journal Psychological Science.
An estimated 1.7 billion people around the world are obese. This means they are also at increased risk of a range of serious disease, from diabetes to heart disease.