Actress Kat Dennings chimed in Twitter.
"Anyone reacting to @@billieeilish having a normal body has to take a hard look at themselves," the Dollface star said.
"As someone who looked exactly like that at her age, it'd be nice for this unhealthy nonsense to f*** right off."
Eilish has been vocal about body image criticism before, and debuted a video clip about the subject when she was on tour in March.
In the clip, she lowered herself into black water and is seen removing her to top, before she launched into a powerful speech about the criticism she has experienced.
"Some people hate what I wear, some people praise it. Some people use it to shame others, some people use it to shame me." Eilish is known for wearing loose clothing.
She added: "If what I wear is comfortable, I am not a woman. If I shed layers, I am a slut. Though you've never seen my body, you still judge it and judge me for it. Why? We make assumptions about people based on their size. We decide who they are, we decide what they're worth. If I wear more, if I wear less, who decides what that makes me? What that means? Is my value based only on your perception? Or is your opinion of me not my responsibility?".