Why are fat people fat? The flip answer -- "because they eat more, stupid" -- just garnered some respectable academic support last week with the publication of a U.S. study that had looked into the question of why the French, with their famously high-fat diet, are still noticeably slimmer than Americans. Given the spreading alarm among health experts about the ballooning global obesity problem, the findings are worth taking to heart.

Even though French people eat a diet rich in cheese, cream and other fats, says Dr. Paul Rozin, a psychologist at the University of Pennsylvania, only 7 percent of them are clinically obese, compared with a whopping 30 percent of Americans. After comparing French and American foods, restaurants, cookbook recipes and even consumption styles (fast vs. slow, say, or eating three set meals a day vs. constant "grazing" on snacks), Dr. Rozin and his team concluded that the key difference lay not in what people ate as in how much.

When they compared the weight of portions served at similar restaurants in Paris and Philadelphia, for example, the researchers found that average portion sizes in the French capital were 25 percent smaller than in the City of Love. And lest Americans think Asian food is less fattening, they had better take note: Servings in Chinese restaurants in Philadelphia were found to be a mind-boggling 72 percent bigger than those in Parisian Chinese restaurants. No question about it: A massive plateful of General Tso's Chicken with rice is going to pack more calories than a nouvelle-cuisine-size portion of coq au vin. The same applies to quantities of food served at home, where, again, the American trend was toward "more is better."