I have heard that if you're over a size 18 then it's 2 stone to drop a dress size, then when you're an 18 and under it's 1 stone to drop a dress size.
In the past I lost 4 stone and only just lost a dress size going from a tight size 24 to a loose 22. But I'm an apple shape so all my weight is on my middle so I have to lose loads for it to make a difference.