Cicciabomba (1982)

Miris Bigolin is a vastly overweight high school girl who is relentlessly bullied by the "hip" boys and girls at her school. Her only escape is a nighttime call-in program that she hosts on a local radio station. Just when her humiliation has reached its nadir, she wins a trip to New York City, where she is immediately pounced on by Baroness von Kemp to showcase her amazing weight-loss formula. After several weeks of diet and exercise, Miss Bigolin suddenly discovers she's lost more than 80 pounds, and after fulfilling her contractual obligations to promote the weight-loss product, she returns home to exact revenge on her tormentors... primarily the narcissistic Mirko Mariani, who has compounded his crimes by ditching her sister, Deborah. Miris and her pals kidnap Mirko's current fiancé, Simona (the wealthy mayor's daughter), and convince Mirko (who fails to recognize skinny Miris) to dump Simona and marry Miris (who is posing as Baroness von Kemp's daughter). Once they get Mirko to the alter, the plot is exposed, and Mirko is forced to marry Miris' sister Deborah after all. Of course it wouldn't be a happy ending unless Miris gets hooked up with her English shag-mate from New York, which she does.