Sizing has definitely changed through out the years. When I was a teen (many many years ago) I found a really cute pair of jeans shorts that had belonged to my mom before she had given birth to me. Yes, she hangs on to things for years. They were a size 14 and she had weighed 120lbs. when she wore them. As a teen I was 130 and a size 14. I could get them on, get them to zip but just couldn't manage the button. I have never been able to get into them. I am now much, much heavier and my pant size says 14...I seriously doubt it is accurate. Actually, who am I kidding. I know it isn't accurate. It's not different then the portion sizes at restaurants today. The small sodas at fast food restaurants today used to be medium/large back in the 80's. I don't know why manufacturers do such things. I can't imagine they are fooling many people.
If you have a big butt like me, its even worse. I can fit into a size six American Eagle which fits fine around my waste but too tight around my butt. Its pure evil. It changes between companies too.
Brands definitely vary. I wear between a size 7 and 9 most of the time. I have found some brands I fit as small as a 5...it just depends.
The other day, I tried on a normal size 8 corset. It didn't fit me, yet by some miracle the size 6 "petite" fit me? P E T I T E !!!!! Very very, weird.