Really interesting! But doesn't this mostly track body weight? Body weight has been going up.

If you have a cultural expectation that "8-10" is sort of a small-side-of-typical size, but average weights go up, you can either decide measurements are fixed (like normal people do when measuring anything else), or decide that "8-10" is the small side of typical and shift the dimensions to fit how big people actually are.

It seems mostly like the U.S. has just done the latter: ~25 pounds at roughly the same height corresponds to a change of about three sizes.