“Fat Girl,” seemingly more innocent, at times almost like one of those sophisticated French movies about an early summer of love, turns out to be more painful and shocking than we anticipate. It is like life, which has a way of interrupting our plans with its tragic priorities. True, Anais (Anais Reboux) achieves a personal milestone, but at what cost? The movie takes place in a summer resort area. Anais and her sexy 15-year-old sister, Elena (Roxane Mesquida), are vacationing with their mother (Arsinee Khanjian). Their father (Romain Goupil) is a workaholic for whom the family is just one more item on his to-do list. Elena attracts the attention of the local boys, and her overweight kid sister looks on with smoldering jealousy: Anais at 12 is smarter and in certain ways more grown-up. She is eager for a sexual experience, although she has little idea what that would entail (in one sad-sweet scene in a swimming pool, she imagines a romantic rivalry for her affections between--a pier and a ladder). In another scene, she eats a banana split in the back seat while watching Elena necking in the front.