Ok, you wanted my view, so sure if the GDP is really low, people starve, they are underweight, now if the GDP is high people have a lot of money which equals a lot of food and they get fat. But it ain't that easy.
I live in a country with a GDP of 700 dollars which makes it one of the lowest countries (high in the lowest category but still lowest). People here are not starving though (mostly), but they eat a very unbalanced diet, because carbs are cheap and veggies are expensive, so people eat a diet based on carbs and fat. Not surprisingly people here are obese, not everyone, but a lot of them. The same happens in Mexico, the GDP in Mexico is way higher than here in Ghana, still they are not up there with Europe or the US, Mexico has a very large number of obese people. Why? Because junk is cheap and gyms are expensive.
When you are poor you will have to settle with what you can pay, and you will be working a lot, so not a lot of free time. If what you can pay is junk full of fat, and your job is to sit all day selling stuff, then you will get a lot of cals in and not so many out. If you are in a rich country only have to work 8 hours a day and can afford nice food then you can afford to be skinny too, because you will be able to pay for a gym, a trainer, workout DVDs and a DVD player.
Also in Ghana we have almost no anorexia bulimia problem. I mean sur ethere are some kids who do have ti, but the % is way lower than in Europe. Why? I am guessing when you are poor you do not give that much importance to your looks, it is more about survival. If survival renders you chunky, well you know what count your blessings, you had enough to eat at all times!
If the GDP is ridiculously low (Niger has a GDP of like 130 dollars, I think) then more people are starving, more people fall into the undernourished category and more people are "skinny".
So while the blogger pointed out something interesting with Japan having more skinny people, it does nto surprise me, it is in a way easier to be skinny if you have more money than if you have little!
Camy