Roxane Gay discusses gaining weight as a form of protection after sexualized trauma in her extremely beautiful book Hunger: A Memoir of (my) Body. TW for sexual assault even for reviews of the book. If you are up to reading it, I cannot recommend the book highly enough. It is a meditation on being fat as a woman, and she spends a lot of time on exactly this idea you are talking about, gaining weight as a form of protection/invisibility in a world of sexual violence. Here's a link to a short review on the Guardian and the book on Amazon.
I have heard other sexual assault survivors say the exact same thing. In fact Roxane Gay mentions this quite a bit in this book: https://www.amazon.com/Hunger-Memoir-Body-Roxane-Gay/dp/0062362593 you should take a look... and also see therapy.