i bought this bubble mini at old navy last week for $10, and i was psyched about the great price: but i wore it today for the first time and it kinda sucks. there is no elastic at the top (only at the seam where the fitted part meets the bubble part), so within 20 minutes of putting it on, it was falling down. (granted, i have no butt to hold stuff up, but that whole top portion was draping) at one point during the day (after 30 minutes of wear) i walked 50' from my car and up a flight of stairs, only to discover in the mirror at the top that the entire skirt had fallen down my butt in the back, exposing 80% of my a**/underwear.
i don't think that i would have been able to go down a size b/c the size i got is as short as i'm willing to go (smaller would be obscene). it's a pretty short skirt -- short on me at 5'2. also, i think a smaller size would have the same stretching issue.
other than that issue, it's a really comfy skirt. i will probably try to put some kind of elastic into the waist (or maybe a drawstring) just b/c it's great to pull on as a comfy thing and wear as around the house/loungewear kind of stuff.