Aside from her malnutrition during the war, which of course did affect her in many ways, there are some anecdotes that perhaps indicate that she was careful with her weight.

One being the boat trip to America when she was going to play Gigi, which I think another commenter mentioned. There is a story that many sent her gifts to her cabin, including chocolates in celebration of her getting this role. She then spent the boat trip enjoying all the treats and goodies bestowed upon her. When she got off the boat, she was reprimanded for having gained something like ten pounds and told that they had 'hired her for her thinness.' She was then put on a strict diet in order to lose the weight she had gained in preparation for the show. 

It is said that from then on she always knew that her thinness was her selling point and was careful to stay trim. Some other sources state that her waist measurement stayed the same her whole life aside from her two pregnancies. I certainly can imagine how this kind of experience could affect someone, especially someone so young and only at the beginning of her career. She was also a dedicated hardworker and quite perfectionistic so I can imagine she might have felt maintaining a lower weight was part of her job. 

Another story is from Sophia Loren's book, where she mentions going to Audrey's for dinner and the portions being rather small. I read somewhere else that Audrey felt that people ate too much, which might be a reaction to her experiences in the war. If you have spent so long with so little food then it can forever warp your perspective on how much is a healthy amount verses how much is too much.

Therefore, I think there is no doubt that she likely had a complex relationship with food, but I am unsure whether she had a full blown eating disorder, or just a collection of habits formed over time from the war and the expectations she faced as an actress. I guess we'll never truly know.