The less you have to lose, the less pounds it takes to drop a dress size generally.
When I first started, I had 4 stone to lose. The first 2 1/2 stone I lost a little over one dress size. Then after that it was 2 stone lost, 2 dress sizes lost and the last 1/2 stone made me a dress size less. So it did average out to be about 1 dress size per stone, but the inchloss did go faster as I got closer to my goal weight.
Right now, it takes about 10 pounds to lose a dress size for me.
(my ticker needs updating, but as I'm not seeing my CDC till Tuesday, I'm waiting till then!)