Posting a photo of Marilyn Monroe, the 26-year-old wrote, "To all the girls that think you're ugly because you're not a size 0, you're the beautiful one. It's society who's ugly,"
She added, "Thanks to my fans who love me no matter what, and know the meaning of real beauty and compassion. I really love you."
Gaga admitted she's gained 25lb (11kg) after breaking her strict diet and indulging at her dad's Italian restaurant in New York.
"I really don't feel bad about it, not even for a second," she told radio host Elvis Duran last week.
