I was outside most of the day today, so I did of bit of unscientific observation. I'm afraid I just don't see this. People are either normal-shaped, or fat. Roughly 70-80% are normal. The fat ones tend to be older people, which is to be expected in a way, with the occasional younger outlier; perhaps 10% of the total population.
Skinny-fat, specifically, I saw very few, and they were mostly men, not young girls. The defining characteristic of 'skinny-fat' is a spare tyre around the abdomen, or a floppy paunch (however small, it'll be there). Without one of those two, you're not skinny-fat; you're just skinny.
The exercise aversion results in poor musculature, not really fatness per se; hence the knock-kneed girls or apple-shaped men with toothpick legs (Dilbert-shaped, basically). Having said that, I wouldn't say Taiwanese people were more or less sedentary than (say) Americans.
I've noticed this in the UK. Most girls up to about 16, and boys up to about 20, look perfectly OK regardless of their diet. Once they hit their 20's, they turn into pasty, flabby wrecks with terrible skin. Since this is men as well as women, it can't be a childbirth thing - it's more likely to be growth-related. When you're a teenager you're growing extremely rapidly. Your body can use all the calories you can throw at it. However, this growth ceases suddenly at about age 18, plus or minus. At that point, a junk-food diet has its adult result, ie., instant obesity.