Testosterone increases all aspects of sexuality in women.  That is why
the testosterone patch has recently been in the news.  Testosterone
increases sex in women.  Sexier women make more babies; they increase
in percentage faster in a population than lower testosterone women.

Testosterone is higher in obese women. "Only %free T was higher in
obese than in nonobese women (P = 0.02)." (Nutr Cancer.
2001;41(1-2):47-56).  

Higher testosterone women increase more rapidly, increase in
percentage with time,  than lower testostrone women.  (I think this is
the source of the "secular trend," the increase in size and earlier
puberty in children.)  This also increases the percentage of fat women
in our population with time.

Obesity is not causing other problems; obesity is caused by the same
thing that is causing a lot of problems in other areas as well:
excessive testosterone.