Some chick who does inflation or something.

[Dragon FangX]

I think it helps the fact she is a woman in why she's more capable of doing this. After all, women's stomachs are design to increase in size to great lengths due to the ability to bear children and, in some cases, a great many children at once. Basically she's just slowly been increasing the elasticity of her skin to the point that she's soft and squishy enough that it can expand to great lengths. Wouldn't be surprised if you give her another year or two she might start to look like some of those mega-preg pictures.