Media polarize people. Those who could have some shot of living up to media hype do it obsessively, even risking their lives.
Others simply cannot do it. One day, their body takes off like a rocket and gain huge weight. All in a span of couple of months in some cases. If you are hopelessly out of range, you quit. I noticed that this happens to many women after 30's. After that, they become real obese.
If you don't care about getting ahead or your body physiology puts you hopelessly out of range, you tend to become truly fat.
As a result, we have polarization as you observed.
It is the same with exercise. Those within range do workouts day and night, more than they actually need. It is really panic-driven and obsessive. If you are out of range, you become a permanent couch potato, and develop a real health problem.
There is also a money component I'm convinced.
Poorer women tend to be fatter.
Less social pressure and other psychological stuff....
Some blame it on junk food. I'mm not convinced.
When I was a kid, poor folks were skinny.