Speaking from my personal opinion, Stevie lost a LOT of weight by the time The Dance rolled around and even more by the time she went out on the Enchanted tour. There are some points in The Dance (specifically during Gold Dust Woman) where she looks like the skinny California chick from yesteryear. On the current tour, I think she has put on a bit of weight (not a whole lot), but that's probably from going through menopause. I can certainly understand that and I could care less what size she is, personally.
PS: I also think that her shoulderpads and Frankenboks make her look heavier than she really is.