It could be a combination of both nature and nurture.
We need to find two identical female twins...one who has led a relatively modest sexual lifestyle or possibly has abstained from sex completely. The other female twins needs to have led a very promiscuous and active sex life, whose vagina has been used and abused for a couple of decades.
I'm no scientist, but I would think the promiscuous one would have bigger beef curtains.
My somewhat scientific study of Hustler Magazine's "Beaver Hunt" in my youth seemed to recall a higher percentage/ratio of beef curtains in females that appeared in pictures that were taken in what appeared to be trailer parks/trailers (see wood trailer paneling, dream catchers, beer koozies, and floor bed mattresses).