It's really impossible to say without pictures or even numbers to guess by. Even chunky snakes can look a little triangular sometimes (have their spine ridge prominent), when they get themselves in certain positions.

If she's eating regularly and not gaining any weight, you might want to have a vet check her out for worms...which isn't a bad idea to do regardless, if you don't know her background history. (Pet store ball pythons are almost always imported from Africa and may host any number of parasites)