Last Friday, Ms. Renn arrived for an interview wearing a cosmic glitter Maison Martin Margiela cat suit, a sleeveless cardigan from Preen, Rick Owen’s floppy boots and what looked like a bicycle lock around her neck. For all the appearance of success, she has had darker days, as chronicled in her book, in which she describes starving herself to be a “straight size” model, the industry term for girls who meet the prevailing standard of beauty, which is to say extremely thin.

It was not until Ms. Renn acknowledged an eating disorder six years ago and began to eat normally that her career took off as a plus-size model. While she embraces that label, she also sees it as a means of changing expectations among designers and magazines  and even the public  that models have to look a certain way.

Ms. Renn was born in Miami. Her mother, who was still a teenager, left her to be raised by her grandmother, Kathy Renn, a successful saleswoman of Mary Kay cosmetics with a pink Cadillac in the driveway. Ms. Renn came to know her grandmother as Mom, while her mother, named Lana in the book, remained largely out of the picture until her teens. When she was 12, Ms. Renn and her grandmother briefly moved in with Lana in Clinton, Miss., but their relationship ended with a violent confrontation.

Image In a 2006 ad in Vogue. Credit... Tony Cenicola/The New York Times

She never knew her father. “I don’t have a picture,” she said. “I don’t even have a name.”

When a modeling scout told her she had potential provided she lose weight and shrink her hip size from 43 inches to 34, Ms. Renn saw a means of escape from small-town life in Clinton. On a regimen of Diet Coke and sugar-free Jell-O, she began by losing 28 pounds in three months. By 2002, when she moved to New York at age 15, she weighed 95 pounds and had lost more than 42 percent of her body weight. On her first day in the city, she landed a shoot for Seventeen.

Kathy Renn, now living in Riverside, Calif., where she is the executive director of a nonprofit group, Fuel Relief Fund, said she always thought her granddaughter was in control of her diet and weight. “One thing about Crystal,” she said, “is she is very goal-oriented. When she set her mind to going into modeling, that’s what she did.” Although she followed her granddaughter to New York for several months, Mrs. Renn was not fully aware of Crystal’s weight struggles until she read the book.