If you've ever tried on a vintage dress that's four sizes larger than what you normally wear and found yourself unable to zip it up, you might've thought something like "Holy shit, people used to be tiny! We're all giant horror people now!" But the truth is that we're only sort of giant horror people. Rather, the bigger influence on the difference in numbers is vanity sizing, the practice of moving clothing sizes lower so people feel better about what size they wear.
According to the Washington Post, in 1958, "Women's sizes ranged from 8 to 42. A size 8 woman had a bust of 31 inches, a 23.5 inch waist, and a weight of 98 pounds." But "the measurements that added up to a size 12 in 1958 would get redefined to a size 6 by 2011." Great news! That means that if you're a size 6 now, by 2064, you'll be a 0. And if you're a size 0 today, you'll be a -6, and everyone will complement you on how you don't even exist.
Incidentally, the 1958 measurements were ridiculous in the first place. They were based off of a survey of 15,000 women that was undertaken the goal of figuring out standard measurements for all ladies. But what the survey actually discovered is that lady bods vary so much that they defy standardization. Still, the National Bureau of Standards used survey data to publish... well, standards. And as any person who has ever gone shopping can tell you, clothing companies have been blatantly ignoring those standards ever since. So one thing is certain, no matter what our body type is or how much vanity sizing changes things: we'll never be sure what size to grab off the rack.
