I think environment, society, ease of access, lack of self-control is a major factor. I don't buy the genetics being a *major factor. I've studied this a lot myself, and I've worked with many nutritionists since I had UC ten years ago, which is now cleared up (Thank the Lord).
Genetic pre-disposition to certain *body frames/height is real... not obesity...
So, in addition to everything else ...
1) more people workign desk jobs (sedentary)
2) higher stress due to the pace of life (artifically created) ... leading to issues with cortisol
3) food ... much less obesity in Europe, where they have banned a lot of the garbage that's sold in the US, various GM things, chemicals, high fructose sweeteners (heavily restricted in Europe) ... Europeans also work a lot less and live more laid-back lifestyles
4) "Wheat Belly" -- fascinating book about how US Big Agra genetically altered wheat starting in the 1950s and abotu how it's led to the obesity epidemic (this was before modern GM, but they still did a huge amount of breeding, looking fro higher profitability, greater yields, insect resistance, longer shelf life -- this led to higher gluten levels in bread and contributed to the epidemic of gluten sensitivity)
So the one individual who went to France and ate large amounts of food whilie gaining little weight (just one pound) ...
checks all these boxes
likely more walking involved visiting a foreign country, probably more relaxed ... on vacation, food much different with all the garbage contributing to obesity greatly restricted if not banned, etc.