"They (the producers) wanted to see me in a different light, as a heavier woman," the 40-year-old singer, who weighed about 180 pounds at her heaviest, tells Us magazine in an issue on newsstands Friday.
"The timing didn't quite work with trying to finish my album, and unfortunately I wasn't able to do the film," she is quoted as saying. (The role went to Mariah Carey.)
"So I found myself having to take this weight off a lot quicker than I had planned."
Jackson says she packed on the pounds by eating whatever she wanted.
"`Ooh, that looks good, give me that whole slice of cake!' That was the fun part," she says. "(Eventually) I would look in the mirror and I wouldn't recognize myself."
The 5-foot-4-inch Jackson says she got back into shape in four months by eating "nice, balanced meals" and working out with a trainer. Her longtime boyfriend, music producer Jermaine Dupri, was a motivation.
"I've never had someone love me for me, so unconditionally, the way he does, regardless of my size," she says. "He was never any different toward me."
When asked about reaction to her plus-size body, Jackson says, "People didn't treat me differently to my face, but I'm sure they had their whispers behind my back. But I didn't feel bad about myself at all. It was part of my job."
Her latest album, "20 Years Old," is slated for release Sept. 26.
"I feel good," she says. "It wasn't until I saw a picture of myself, just recently, actually, that I thought, 'Wow, you really did lose a lot of weight!"'
She adds: "For now, it's just nice for me to get back down to where I feel more comfortable with me."