I suppose the question is … are you looking for reassurance, or do you want to do something about it?
It could be that you look perfectly fine and you’re just uncomfortable about being “different”. We can’t see you, so it’s hard to tell - and it isn’t clear exactly what you feel insecure about. You mention “very fit, has nice hair and skin and is very petite” - so I’m guessing you feel you can’t apply those descriptors to yourself?
It’s true that most people here look a lot healthier than people in the West, and they age better. But there’s nothing special about Taiwanese people - they’re just “normal”. The reason, IMO, is really very simple. Western diet is appalling. It’s barely even food. 30-60% of the European population have some degree of metabolic disease. Of course it varies from country to country, but if you look in the mirror and can objectively say “I don’t look very healthy, and I don’t like this”, the reason is almost certainly diet-related.
Westerners usually don’t realise how terrible their food is. I certainly didn’t until I moved to Taiwan - and even then, it took me some time to figure out what the problem was, and what people were doing here that was different/better.
If you’re insecure about feeling “bigger” than Taiwanese people, or if you feel that you have skin/hair problems, this is something you can easily correct. For example, does you skin appear blotchy or puffy? Do you feel like you’re holding water (press the skin on your thigh and see if it springs back immediately, without any visible dent)? If so, you can fix this.
Have a look at what you’re eating. Is it mostly home-cooked vegetables, meat, eggs, dairy? Or … do you eat a lot of bread, potatoes and pasta? Breakfast cereals? Sweet snacks? Things-in-packets? The typical Western diet looks like the latter. Mine certainly did, and I looked terribly unhealthy at your age.
Nope :). Guys from Western country generally don’t have much reason to feel insecure in Taiwan, and it’s mostly men here on Forumosa, so you’re probably not going to get many helpful replies on this part of your question!