A healthy diet, for one. The US is a country where it's more expensive to buy a salad than a burger - not so here in France. Fresh market produce is still the norm, even though that is changing as processed foods are slowly being introduced. Eating small healthy portions, and a general lack of snacking, including sodas (usually limited to parties or special events rather than a daily drink), has meant that they maintain a healthy weight. It also means clearer skin, seeing as how greasy foods lead to pimples.
Even lower income families benefit, seeing as how they have easier access to markets and fresh produce than to fast food and burgers, which are overpriced by any standard.
(This does not count as Salad. Seriously, Jello deserves no place in one.)
Fast food is a rarity (though still present) meaning that you don't have the smell of grease in the air. I tend to notice this when I visit family in the states; some areas just smell like fries and fried food. They, of course, can't smell it, seeing how it is the norm in some places. But with one fast food place per town instead of per street, you don't breathe in all those greasy odors. This reduced the urge to snack, and also leads to clearer skin. Yep.
I don't think I've ever seen fried chicken or chicken tenders here in France. The French don't tend to deep fry anything, at least not by american standards. Donuts are rarities, and mozzarella sticks, surprisingly, aren't too available either. Fried foods just aren't the norm.
Fashion matters. You will never see a French person wearing sweats. Yoga pants outside of home of the studio? Really? Leggings as pants? Seriously? Not happening. Jeans are common among men or teens, but not amongst women, who tend to dress more proper. People tend to invest in clothes, seeing as how cheapo clothes shops are not prominent. People just put a little more effort into it.
People walk places. Towns are designed so tat everything you need is within walking distance, and there is no room for cars. So people walk. They walk anytime they need to go anywhere, which means that they have a minimum of daily exercise. Gyms are starting to gain popularity, but they're not as popular as int he US. Most people just tend to get enough exercise by doing their daily walking or chores.
Things in the USA tend to be more sugary as well. Compare coca cola from the US and from Europe (even Mexico!) and you would be surprised. I tend to not drink much else than water when I visit the States, since everything is too sweet for me. Same goes for foods: things that don't tend to be sweet here at home tend to be too much for me in the US. Even potato chips: compare barbecue in both countries, and you'll know what I'm talking about.
Taste a French pastry. It's sweet, but it's not pure sugar. The america adaptation of an eclair sends me running for a glass of water.
Let's compare an average breakfast. Both countries have cereal (though guess which one has more sugar) and both have fruit juice. The 'typical' american breakfast consists of eggs, bacon, toast, sometimes waffles or pancakes, and in other cases, even bagels. In France, baguette with jam is quite usual, along with plain black coffee (remember, Starbucks still has not hit). Maybe nutella instead. If you're really up for working, crepes are fantastic, but they take time... Again, most french breakfasts never hit the pan. And forget premade breakfast muffins; non-existant. A croissant or a Pain au Chocolat is a real treat.
So, the secret to bombshell Europeans? Nutrition and exercise are their everyday habit. Sugar is not abused, and fatty foods are almost absent. Health comes from within, and so does beauty. So eat well, and keep walking. Because the US has some catching up to do.