Comparison between the American diet and the French diet, from Wikipedia under The French Paradox:

French diet comparisons

In his book, The Fat Fallacy, Dr. Will Clower suggests the French paradox may be narrowed down to a few key factors, namely:

• Good fats versus bad fats — French people get up to 80% of their fat intake from dairy and vegetable sources, including whole milk, cheeses, and whole milk yogurt.
• Higher quantities of fish (three times a week).
• Smaller portions, eaten more slowly and divided among courses that let the body begin to digest food already consumed before more food is added.
• Lower sugar intake — American low-fat and no-fat foods often contain high concentrations of sugar. French diets avoid these products preferring full-fat versions without added sugar.
• Low incidence of snacks between meals.
• Avoidance of common American food items, such as soda, deep-fried foods, snack foods, and especially pre-prepared foods which can typically make up a large percentage of the foods found in American grocery stores.

Clower tends to play down the common beliefs that wine consumption and smoking are greatly responsible for the French paradox. The French diet tends to cause Americans to lose weight while visiting even if they are not wine drinkers. While a higher percentage of French people smoke, this is not greatly higher than the U.S. (35% in France vs. 25% in U.S.) and is unlikely to account for the weight difference between countries.
Mirreille Guiliano, author of the #1 bestseller French Women Don’t Get Fat, agrees that the weight differences are not due to French smoking habits. She points out that the smoking rates for women in France and the US are virtually identical. Guiliano explains the key factors to the French woman’s ability to stay slim as:

• Smaller portion sizes
• Savoring food to increase the feeling of satisfaction, choosing a small amount of high quality food rather than larger amounts of low quality food
• Eating 3 meals a day and not snacking
• Taking in plenty of liquid such as water, herbal tea, and soup
• Sitting down and eating mindfully (no multitasking and eating while standing up, watching TV, or reading)

Emphasizing freshness, variety, balance, and, above all, pleasure