I think it's completely person to person... 10 pounds on a 5'10 person could only be one size... but 10 pounds on someone only 5 feet tall could be 3 sizes. And everyone loses and gains in different places. So I wouldn't say there is actually a number of pounds that equals a jean size.
I don't think it's pounds more like inches lost that make a difference in sizes. Example if u go to the gym & are weight training you're losing fat but gaining muscle & muscle weights more than fat so you're getting leaner but on a scale it might appear as if u gained weight. I know it's confusing! I never weigh myself just go by how my clothes fit to see if I lost or gained. Also your body type makes all the difference.
Also, besides height is body build. At my SKINNIEST and I would have to be WAY skinny, I would be a size 12. For me, 20-25 lbs equals a size. I currently weigh WAY too much and wear a size 20, when I was 30 lbs lighter I wore an 18 and I needed to be like 50 lbs lighter to wear a 16.
Doctors IDEAL weight for me is 160-165 lbs and I'm 5'6" tall and that weight would make my VERY lean. I got to around 185 a few years ago and my MIL who always thinks I'm too fat actually said I didn't need to lose any more and she was SHOCKED at how much I weighed. She weighed 170 and felt HUGE and wore the same size as me at the time a size 14 and while I was mostly lean and muscular (aerobics daily and running 3 miles a couple times a week) - just had 20 lbs to go... She looked chubby and she's 5'7", but her ideal weight is about 130. So, 20 lbs overweight versus 40 lbs overweight just because of our frame differences.
Of course, I would KILL to be at 185 now... but I'm starting, that's what counts, right?