Just start by giving smaller servings. Really, the size of servings in US is ridiculous, who is supposed to finish that?
This is anecdotal but I was just in France recently and the portion sizes were comparable to most urban areas in the US that I have eaten at (obviously it is not comparable to the rural areas engorging Golden Corral). The big difference is Europe seems to eat a lot less meat. The meat portions were much smaller but replaced with carbs and alcohol. As an American I felt embarrassed that I could not finish half the Mic…
In fact, I struggle to think of a French meal that isn't animal product-centred (I'm including fish, eggs, snails etc.)
The biggest difference is they have smaller portions and way fewer carbs. You won't go out for breakfast in France and be faced with a huge stack of pancakes with hash browns and toast.