Correlation is not causation. Manhattan has many thins because thins are more likely to make more money and get better jobs in high-standard-of-living areas due to commanding more respect. (Which they mostly do not deserve, being mainly the beneficiaries of good genes, but I digress) Also, most people move out to the ‘burbs to raise kids, so Manhattan is strongly weighted towards the under-40 set.

> any other country

OK let’s pick Germany. (I’m German.) Most Germans are thin because they don’t eat many carbs at all. Orange juice is expensive and piss-poor compared to sugary American OJ. Breakfast typically consists of unleavened-style breads, mustard, and meats. Gas is super expensive so everybody walks or bikes. But guess what? Obesity is steadily rising there too! Gosh, I wonder why, with all that walking and biking? Wait, I know… They just need MORE walking and biking, that must be it!