My theory is that the larger you are, the more you squeezed into those clothes to begin with!

That's me anyway!
I've bounced around from 298 to 243 in my adult life and only have gone from a tight 28 to a tight 24. Really? That's all? Yup.
Okay, based on that math (20 pounds = 1 size). That means to get into a size 18, I'm looking at 100 pounds lost total from 298 to be 198. That fits in with my senior year of high school weight and pants size. So, for me, it's pretty steady.
By the way, I am 5 feet, 8 inches and hourglass shaped (big boobs, no butt, small waist but lots of belly).