How can I increase my waist size?

I'm fed up, im a very small girl I'm 21 but people always think I'm like 14. My waist line is so tiny I can never wear anything decent without holding it up 24/7. I've tried weight gain shakes and I eat a lot of calories a day anyway tbh. What can I do? Or is it just genetics.
How can I increase my waist size?
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