The 79-year-old comedian mocked Adele's weight during a February "Late Show" interview in which she told David Letterman that the British songstress should add "fried chicken" to her song "Rolling in the Deep."
Rivers also talked about meeting Adele: “She kept saying, ‘my throat, my throat, I don’t know if I can swallow’ and I said oh, yeah, you can swallow. There is no question."
"It's not mean," Rivers told HuffPost's Marc Lamont Hill on Friday. "She's a chubby lady who's very, very rich, and she should just calm down -- or lose weight!"
"She wanted an apology, so I took an ad out on her ass. I said, 'You are not fat.' And then I had room for a lot of other ads," Rivers joked.
"Adele is beautiful and successful and has what, $100 million?" she added. "Let's face reality: she's fat!"
Rivers has built a career making caustic comments about celebrities. In February, the Anti-Defamation League condemned an on-air joke about Heidi Klum's low-cut red carpet look.
"The last time a German looked this hot was when they were pushing Jews into the ovens," Rivers said on an episode of "Fashion Police."
The ADL's Abraham Foxman called the crack "so vulgar and offensive to Jews and Holocaust survivors, and indeed to all Americans, that we cannot believe it made it to the airwaves."