Genetic issues have always been around, but the rise in obesity is strictly modern. Throughout eons of history, people have been at a severe calorie deficit. Your body has many amazing background processes to help you survive famines (your body will try to retain as much fat as possible when starving over longer periods of time), avoid accidentally killing yourself due to excess calorie burns while foraging (your body builds in an automatic efficiency curve into repetitve exercise to conserve calories), and even some genetic changes for those that endured exteme famine conditions, which were passed down between generations after calamities like the irish potato famine, making people more likely to survive. These are great during civilization collapses, but really bite us in the ass in modern times.

However, the rise of ultra processed foods (UPFs) and other calorie dense foods make it extremely easy to take in far more calories than one could ever burn though exercise alone. As more jobs transition from labor intensive (bricklaying, farming, digging trenches, and laying roads by hand and pickaxe), we have created a more sedentary lifestyle at the same time, compounding the issue.

We definitely need to factor in larger people into stuff like biking, but biking alone will not address the root cause of the problem: 1. the proliferation of UPFs coupled with their low costs, 2. a sedentary lifestyle due to cars and office jobs, and 3. the collapse of third places where people can hang out, swim, play outdoor games, sports, etc.

Nip those three problems in the bud, and you improve the health outcomes for generations of people.

Stuff like urban density, lowering the cost of healthy foods while improving signage on UPFs, making it easy to walk, bike, bus, or commute via rail instead of drive, and improve free or low cost social spaces will help. :)