Fat doesn't necessarily make you fat. It only does so with a very specific confluence of factors:
1) Sugary drinks (as several people pointed out above). Continually topping up with a Coke (or whatever) is guaranteed to make you fat - not just because the sugar itself has nowhere else to go, but the ensuing insulin rush forces any other excess energy into fat storage (including whatever fat you've eaten).
2) Highly-processed grains. Americans are endlessly gorging on squishy white bread (thanks, USDA; 6-11 servings a day, really??) which has the same metabolic fate as sugar.
3) Sedentary jobs. When you eat starchy food, energy enters your bloodstream at a peak rate of about 1000kcal/hour, or 1100W, implying a matching mechanical power output of 200+ watts. Nobody works that hard these days.
There's no way of guessing what other factors in processed food might be responsible - there are all sorts of chemicals and non-foods (eg., soya derivatives) that might act as endocrine disruptors, for example, but who knows?
Here are the USDA guidelines.
If you wanted to write guidelines designed to make people fat, this is what you'd write. It's the most appalling pile of unscientific tripe you'll ever have the misfortune to set eyes on.
Taiwanese people simply don't eat much sugary food, and surprisingly little rice compared to the American equivalent starch intake. They cook at home instead of eating from a box, or at least eat at a restaurant where the ingredients are just meat and vegetables.The average home-cooked meal involves a small bowlful of rice and plenty of vegetables. A bian dang tends to be heavier on rice because it's cheaper; you'll notice people who live on bian dang do get a bit squishy.