I voted belly, but my list is kind of different. Mainly because I like a woman with a nice pot belly and nice boobs.
For my list:
1. Weight gain, but only an apple shape.
Where most of the weight gained is in the belly and boobs. This picture provides an excellent example:
gomyugomyu.deviantart.com/art/…

2. Pregnant women. For the sake of this poll I would consider it a form of body expansion.
A close second, but mainly because you get a nice belly with nice boobs to go along with it.

3. Boobs
4. Hourglass
5. Butt

I never liked giantess, but sometimes I get in to Koonago (shrunken women).