@ossobuco,
Not to mention the changes in body size.... OK won't go there.
Seems to be no consistency in size designation. Vanity, vanity , all is vanity.
Many years ago, I weighed around 120 lbs and wore size 8 or 10, usually 10, never liked tight fit. I wore moderately priced clothes, shopped at Macy's, Ohrbach's, May's, Aunt Sally's and Uncle Will's. I went with a more affluent friend to Sak's 5th Av. She was buying, I was playing. I tried on many very pricey dresses, and immediately went from a size 10 to a size 4.