Apart from genes, i think it is also highly due to the fact that curvier/fatter women have to fight for almost everything in their lifes :

- getting a good man
- getting a good job
- getting respect
- being taken seriously
ect ect

So they devellop an ability to use their brains to get where they want and with years, they devellop quick ways of getting there.

Whereas " waifs" rely mostly on their look and it works, they actually get the top men, the top jobs and all the rest.

Problem is, since they don't have to " over- think" to get anywhere, they intectually stagnate into late teens.

They never have to rethink their ways or reevaluate their functionning systems.

Until they are 50, left for a young one, the kids are gone and they don"t have enough money to maintain their lifestyles.

In the same position, a fatso will fight, because she knows how to, she will carry on.

A waif will either find another man or live in a very small dive in a very chic area and spend all she has in trying to maintain the illusion.