I haven't PSP is such a long time so I don't know how much help I can be, although I think Zorro uses PSP, might be worth getting in contact with him.
I vagley remember the technique I used back in the day but I don't think it'd work very well for getting to the same size but I'll share it with you all the same.
1. Make a selection of the area you want to make bigger. (In this case the belly)
2. Copy the selection
3. Expand the selection by a small amount. (I'd say between 1 and 5 pixels, 1 would probably be better)
4. Paste into selection
5. repeat from step 2 until you reach the desired size.
It's not great but as you're only making the pixels bigger, so you'd need to mess around with the smooth tool, or the blur tool, which can make things look worse. Other than that there's always the donor route, there are some good tutorials in the tutorial section of the BEA that deal with donor morphs, as well as how to do shadows and they're all free!
At this point I'd say that your major point of weaknes is the lack of shadows. You're blowing these ladies up to at least twice their previous size, this would cause the shadows and highlights on the skin to change. Think of a ball being lit from above, the under side is going to be really dark while the top is going to be bright, you have to use this same thinking with the bellies, obviously looking at the lighting in the scens and trying your best to mimic it.