The truth (as everyone knows) is that people are getting fatter, and manufacturers are moving the goalposts to fool the public into thinking those extra 20lbs equate to 'heavy bones', or 'muscle', or 'buggered scales'.
Sometimes they do.
Mostly they don't.

With no size regulation, shops can choose just about any measurements and label them an 8, 10, 12 etc.

America has to be the worst for this.
Your size 6 is a 10 in the UK.

So you're happily walking around thinking 'well I'm a size 12, same as Marilyn was, and look how many men loved the way she looked'- it's a lie.
A US 12 is a UK 16, and a 16 does not equate to that slim curviness of the blonde bombshell.