For me, it also takes about 10 lbs to get a change in size. I am 5"7. When I weighed 158-160 I was starting to have to buy size 12s in all of my clothes but I never really got jeans that were 12s. I just looked for "big" 10s. When I got down to 150, I was definitely fitting well into a size 10. Now that I am dipping below that, I am going between 8s and 10s. If I got down to 140, I defintely be in an 8 and probably looking at 6s.
For the record, I think pants are getting bigger and bigger. Many, many, many years ago, when I was weighing in the 130s, I used to consistantly wear size 10. Sometimes now, though, I will have to by 6s to get the pants to fit right (Ann Taylor comes to mind). It is absolutely ridiculous. The waist on clothing is horrible too. I am wearing a skirt today from the Gap (size 8) that is so big in the waist I have to wear a shirt over it. I have never had this problem until last year. I realize that they are making clothes to fit "on the hip" but why do I want a skirt like that? My pantyhose go up about 2 inches above the waist of my skirt.