Your hips widen when you become pregnant. Some women are fortunate enough that their hips revert to their original size after giving birth - other women are stuck carrying the wider hips for the remainder of their lives. Other than that, the size of your hip bones depends on the size of your skeletal structure, but gaining or losing weight can make the measurement of your hips larger or smaller, as already stated by sheep.
They do increase if you are overweight. But you can't get below a certain circumference, because you have bones there.
Look at this clothes sewing measurement charts table. The small sizes (4 to 10 in American sizes) have no great difference, the plus-sizes (20 ++) do have waaaay larger circumference. http://funadvice.com/r/bltsops0cr2
