If you're interested in this, you can also find work by Gary Taubes and blogs like Mark's Daily Apple.

It's true that we move less and eat worse processed food (at least in the US - I know that even flour is not actually the same in the US vs EU, the processing is different and it very clearly tastes different to me - I am a bread person) but that's not the whole story. You can stay lean in the US too, if you did the same things people do in Asia, even without any activity. I've also researched this a lot and my top points for it are:

1- "Normal" people should not be snacking all day long. I don't even know why snacking is a "thing" in the US that many people seem to think is necessary. Snacking is a culture, and you will find it doesn't exist in places like France and many parts of East Asia. In the US it just fuels carb addiction, imo.

2- I don't believe humans can't eat any carbs at all. The key is that despite eating rice at every meal, Asian cultures eat a very balanced meal 3 times a day, with even fish and soup for breakfast, and this isn't punishment to them, they genuinely enjoy and appreciate a healthy meal. France is full of (for me) world's best sweet delicacies, yet they realize how much of it one should be eating (a tiny amount by American standards) and most do not go overboard regularly in their life. Italians eat a very moderate serving of pasta, nothing like what Americans think, and despite eating pizza all the time, their pizza is again, nothing like American pizza. Mostly thin crust with lean toppings (no pounds of fake cheese on top) and they tend to eat the pizza in a group - so in effect eating only a small amount per person. I'm saying all of these from my personal experiences, of course obese people exist in all countries.

In short, the key to staying naturally thin (again, from my experience) is partially the availability of good quality, minimally processed food, and partially applying everything-in-moderation without even needing to think about it because it's ingrained in your lifestyle.

Genetics do play a minor role, but as the last decades are showing us, those cultures are also getting fatter due to American food and culture spreading around the world, so it's not like they're immune to it. The same way that white people aren't predisposed to gaining tons of weight if they ate like asians (sorry for the generalization) do. In fact many white Americans report losing a lot of weight without intending to when they move to Japan/Korea.