One other interesting factor: BMI is a rough measure with substantial room for error. Everyone likes to criticize BMI because a lean athlete can show up as "overweight" due to muscle, but the opposite is also true. If you have a lot less muscle then increased fat can show up as "normal".

I would hazard a guess that there are a lot more people with less lean muscle tissue than the 1970s, so BMI can understate how fat people are.