Smoking and illegal drugs are down, Obesity is up. (USA) Is there a correlation?

According to the Nat. Inst. of Drug Abuse, over the last 10 years smoking has declined nearly 50% and most illicit drug use has declined - except marijuana use which has increased. Over that same period, America's Health Ranking says that obesity has increased over 30% and has become recognized as a national problem. Are these phenomenon related? If so, what does that say about us? If not, please explain why not.