The best explanation I've heard is that a company matches the middle of its size range to their average size sold, and then size up and down from there. This has to do with creating patterns and ordering fabrics. So I think it's not necessarily vanity sizing (although I'm sure that happens also) - it's just a reflection of America's average measurements going up, up, up.
But still frustrating. I am wearing size 6 pants today, that I KNOW are about the same as 10s I wore 10 years ago. I never thought of myself as size 6 even when I was at my slimmest (which is my goal, 130 - not counting any teenage stuff
), and here I am wearing them while 10 pounds over goal.