Your ball looks thick, and even a little pudgy, but for her size/age that is relatively normal. She is going to slim down as she gains length, and your current feeding schedule isn't going to stop that from happening. Personally I do not feed my balls or boas any more than once every 7 days, and even keep a sloppy schedule so that every third week is more like 10 days. I do not like to "power feed" my boids, I find they maintain a better appetite and healthy weight without trying to blow them up as fast as possible.
My hogs on the other hand eat smaller meals every 3 to 4 days (I swear they would eat every day if I gave them the chance!)