I think they do this nonsense to make women feel smaller than they actually are.

Smaller numbers are easier to wrap your mind around, and feel good about.

It is all about illusion.

Realistically it would make more sense to use inches but for a big woman that is going to be a lot of inches and even more millimeters.

In Gone with the Wind Scarlett has a bragging right for three counties for having a waist only 17” around while corseted. In today’s size listing anything over size 12 is considered a Plus Size, in most stores.