I can say only what I've casually observed: The average French person is not much thinner than the average American, but there are many more very heavy people in the U.S. than in France. Statistics bear this out, I think. In America, the obesity rate has reached what some nutritionists call epidemic proportions.
But obesity is increasing in France as well, for familiar reasons: lack of exercise, overeating, indulgence in high-calorie, nutritionally empty snacks and fast foods. It isn't hard to understand why many people here see bad eating habits and widening girth as further alarming evidence of the globalization of American culture. I've seen this trend in Paris, kids eating junk food on Metro platforms, burgeoning portions in restaurants, even, on occasion, all-you-can-eat menus, which once would have been unthinkable.
The traditional French style of eating is based on three rules: no seconds, no snacks and small portions. Kids used to learn to eat this way at home and carried the regimen through life. Together with certain less healthy habits like smoking and drinking and the French obsession with appearance, this approach to eating once tended to keep them lean. Exercise for weight control has only recently arrived in France and doesn't seem to be seriously practiced.
As someone who's always had to diet, I hoped that I'd catch onto the French way of eating by moving to Paris. Perhaps I have, to an extent; anyway, I haven't gained weight. The beauty of the three rules is that it still allows me to eats all kinds of foods Jenny Craig would never hear of: buttery potatoes, pastry, chocolate, cheese -- but always in small portions. It seems to me that when you give yourself modest amounts of the things you crave in the setting of a meal, it keeps you from snacking and bingeing. Plus, it makes you happy.