I also think there's lots of confounding variables at play here. TFA says their diet differs from the mainland in several important ways: more vegetables and fish, less white rice and meat, and lower average calorie intake. Both of the first factors are extremely well-documented and uncontroversial dietary improvements, basically in the recommendations of every public health organisation in existence. We also know that being overweight is bad for your health, so as you say, if they're just less overweight than average, nothing much to see there.