So, you've decided to go spend a few thousand dollars on clothes. You're a little heavier than you were last year (it was a rough Christmas) but you'd rather not be reminded of that.
One would think that clothes sizing would be a fairly standard system. It's not. Over the years, as our population has --ahem-- expanded, sizes have as well. A women's size 6 is a lot bigger than it used to be (and a Junior Women's 3x is hardly junior).
As a result, the shopper can tell herself a lie... a story about both looking good and feeling good. And that's the whole point of clothes shopping, isn't it? We don't need a new outfit, we want one. And shopping for expensive clothes is all about changing the way the shopper feels.