Lyn - the bubble hem is elastic, but it's fairly loose so not binding or constrictive at all. These tops are self-lined, which is another nice feature as it gives them weight, structure, and I don't need to wear a silk cami underneath for additional warmth. The elastic joins the self lining with the top material at the bottom and gathers it into the bubble shape, if that makes sense.
I've tried similar bubble hem tops in the past and they weren't self-lined so they'd hang ....sloppily. Kind of saggy. Plus they'd cling to my belly and exaggerate it, rather than concealing it. The self-lining makes all the difference as the top material just glides over the self-lining and no more clinging. Also other similar tops were just too long! They'd fall at my low hip or even longer than that, then when hiked up to the waist, there'd be too much excess fabric. But this one is petite so it hangs at just right length.
This one is a PM as it was the only size left when I was stalking Macy's site for more in this style. It's more generous in the chest and arms, but only just a *tad* longer than the two other ones I have that are in PS. I think either size works for me, just throwing that out there if anyone wants to try these tops: I would caution that Alfani petites do run short -- probably not ideal for petites with a proportionately longer torso.