Ah, my mistake. I've seen nothing "conclusive" on that, and I'd highly suspect that the majority of it is just through reasoning.
Slim / average (not anorexic, not overweight) is indicative of health, which is indicative of both healthy babies and a more careful lifestyle, thus more likely a better mother (biologically! not implying deeper connection here).
Meanwhile, wider hips imply a wider birth canal. Combine the two, and you get the hourglass figure, and a relatively arbitrary (/ personal) choice to use 0.7 instead of, say, the golden ratio. (or e/pi, or any other ratio)