why Asian girls are so skinny?

I (166cm,55kg=5ft5in,120lb)still feel like I am fat and should go on a diet. 

Why???

Recently, I saw a lot of negative comments about kpop idols who gained some pounds. I think compared to the States where I live now, we don't see varieties on model's body size that much. That is changing, but still, I would say most of the models, singers or idols in Asian countries are pretty skinny. I think people still believe "the skinnier the prettier".  That reminds me of my high school day when I truly believed that I should be skinny to become pretty. 

Today, I decided to talk about my experience. Why I have to feel this way about my body and what I want to be.