You took the words out of my mouth, and elaborated far better than I ever could.  But, I think you may have missed something. If the drape is being worn like a toga, then it's front and back cloths are attached to each other at the waist, on the side opposite of the attached shoulder. That prevents it from sliding towards the middle and scrunching as she walks. For the fat one, it looks like they had to cut that stitch so she could fit into it. That's why the fabric is scrunched and folded where it meets the belt. It presumably would stretch across her, but the bottom of the fabric would be lifted to almost crotch level, and that would completely ruin the look. It's the perfect example of why that stitch is so important. Otherwise, I can't fathom how or why they would make the bodice bigger but not scale the extra cloth wrap with it. As far as I know, Ferre was the only designer who's clothes could look fine even when they didn't fit, but he died years ago (to my great sadness, I loved his work).