Obesity is only one symptom within the larger cluster disease called Metabolic Syndrome or Syndrome X. It also includes type II diabetes, insulin resistance, heart disease, high blood pressure, hypertension, etc.
If East Asians deal well with carbs, then they should have low rates of these symptoms. They do tend to be thinner, but that is only because they don't eat loads of food to begin with.
More glucose causes more insulin causes more storage of fat -- but how much fat gets stored depends on how much chow you're eating. If they don't eat many large meals like we do, their higher glucose and insulin levels won't have as much fat to store.
So we need to check whether East Asians or other groups have high / rising rates of diabetes, insulin resistance, hyperglycemia, etc. Those are tell-tale signs of not dealing well with carbs, as they respond directly to how much glucose you're taking in, whereas obesity also requires a high level of food intake to show up.
The other symptoms of Metabolic Syndrome are runaway in East Asia, South Asia, the Gulf states, etc. See this post. Here's just one summary of the Japanese from 2005:
In fact, the prevalence of diabetes, particularly type 2 diabetes, is increasing explosively in Japan, as well as those in the other Asian and African countries. Japan may be one of the most diabetic countries right now. [...] insulin resistance enhanced by change of lifestyle ...
And another from 2002:
Among Japanese men, these changes have been associated with a steadily increasing body mass index (BMI), a well-known risk factor for the development of insulin resistance, impaired glucose tolerance, and diabetes. Genetic characteristics common to many Japanese may also contribute to their higher prevalence of diabetes. The Japanese have a higher prevalence of polymorphisms for at least three genes that code for proteins thought to play key roles in lipid and glucose metabolism: the beta 3-adrenergic receptor, the peroxisome proliferator-activated receptor γ, and calpain-10. The interaction between changes in lifestyle and the ‘thrifty’ genotype characteristic of many Japanese people may play a significant role in the increasing prevalence of diabetes and associated cardiovascular risk in this population.
Again, just Japan, but it's true for all other thin but carb-scarfing countries. They all point to their inability to thrive on a high-carb diet, but you'd only detect that by looking at diabetes, insulin resistance, glucose metabolism, etc., rather than just obesity.