I think the body positivity movement is a reaction to all these people being fatasses. Not the cause of the obesity epidemic. It absolutely makes the situation worse. But the FA's weren't exactly popular until recently with the massive rise in fat people and social media. The cause is probably a mixture of the massive portion sizes (increase 150% since the 70s), added sugar / corn syrup in everything, sedentary lifestyles (48% of people don't even walk the bare minimum of 150 mins a week), and a degenerate culture that doesn't deny itself anything.