Why is 155 too skinny for a 6’4 woman but not a 5’9 woman?

As a 6’4 woman I’ve always been thin. I was skinny until puberty hit me (thank goodness) and I filled out. I got curves, hips and boobs. I’m still thin. I get called flat.
I got told I’m too skinny or I look too thin. My cousin is 5’9” and the same weight as me but nobody says anything to her.

Why is 155 too skinny for a 6’4 woman but not a 5’9 woman?
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