I’m glad to hear the BMI idea worked out, and hopefully your mother starts to feel more comfortable with letting you be the way you want to be. I’m not Asian but I actually face the same kind of issues with my family, especially regarding gender expectations and marriage. And ironically, I have the exact opposite situation that you have but feel it similarly. With me, I know my BMI will indicate that I’m not underweight or overweight, but I know I need to lose a little weight to become more healthy. I want to have a healthier and toner body, and some family members made fun of me for that and gave me the excuse of “Why bother? A husband wouldn’t like a skinny wife.” This always angered me for several reasons (which I’m sure any woman would be upset with hearing) but mainly I hated people making me feel like I need to be changing myself to please others and what they think I should be rather than being happy with myself, and I always told them if marriage means me living with a man who wants to mold and shape me to his desire, then I won’t do it. I’ll only marry someone who can accept me as I am and can be happy with me, regardless of all that.
Anyway, I guess what I’m trying to say is I understand it’s hard to live with expectations you don’t want to deal with, but I like that you’re focusing on making yourself happy. Just try to communicate your honest thoughts and feeling to your mother calmly, and hopefully she’ll understand you better and wouldn’t make you feel this way. I wish you all the best, and good luck!