That's because French dinners are social occasions. They don't eat that much, generally. In fact, one benefit of that style is that there is plenty of time for one's system to react and send a signal when you have had enough. (My late wife was French, so I have some experience of family gatherings.)
I understand that one factor in obesity can be the bad habit of wolfing down food very quickly, as it takes time for the body to react to the amount of food consumed and send a "full" signal. So if one eats fast, one runs the risk of overeating. A slow succession of small courses, in which ones chooses how much of each to take, is far better from that point of view. I doubt that anyone in France (apart, perhaps from Obelix, who is no role model) would drink a whole litre of milk in a few minutes. It would be thought rather barbarous, I suspect.
The French are a lot slimmer than the Americans - or the British.