I really think the more overweight you are, the more it takes to go down a size. I have a friend that was 200 when she started, lost 26 pounds and only went down one size. The belly fat is the last to go which really prolongs going down in size for those with a lot to lose.
Ive lost 19 pounds and gone down 2 sizes, but I am short and didnt have that much to lose. My body gets to a point (from previous experience) that eventually only about 5 pounds is a pant size. Dont get frustrated, you will get there!