I'm voting with the "genetics" group I think.
Have you visited the "Shape of a Mother
" website? It's a collection of pictures of women who have given birth, and you can search the collection by keyword (including cesarean). It can be very healing to see so many different maternal bellies, breasts, thighs, arms, etc. They even have pictures of celebrity bellies that look the same as everyone elses.
I agree that the "Mummy Tummy" book is a huge help in closing any remaining diastasis (which most women have as a result of pregnancy, regardless of delivery method) and it does pull in the tummy a lot! After my c/s I used the exercises in that book and was fairly smooth tummied by a year pp. Same thing after dd2's VBAC birth... about a year of breastfeeding, nutritious eating, mummy tummy exercises, and regular walking and my belly was again smooth. Not a six pack by any means, but no muffin top either!