Years ago, I went from 196 to 160 and lost 2 sizes - which comes out to roughly 20 lb per size.
However, I've now lost 30 lbs and I've only lost a size.
I support others here who've said the more you have the lose, the bigger the weight loss will have to be to drop a size. I'm eager to see how my sizes vs. pounds lost ratio changes as I get smaller.
Or - it could just be that I'm 37 instead of 29 and I was able to do much more intensive work outs back then, so my muscle mass was better.