I would personally prefer it to be localised or endemic, at least at first, just to watch people outside of the fat-obsessed gaining masses go nuts over the sudden quasi-religious followers of gluttony explode their waistlines and flip every societal bodily norm on its head.

I would hit that button, even if it were just to start the religion of fats, worshipping the pursuit of divine immobility. I'd watch with delight as the gym nuts and fat-phobes convert to the cause and follow their deepest urges to become gluttonous zealots and spread further the butter-soaked word of blubber.

I like the idea of extreme fat still being seen as rare and exciting, different but also deeply sensual. If everybody was fat... fat fetishism would probably be the sole haunt of hyper blob enthusiasts! I think the contrast with skinny people is still very enjoyable, so would we'd still need a few sticking around... even if they are just the skinny ones who haven't embarked on their own obesity-glorifying weight gain journeys yet.