Some Discussion Points
What's causing the greater incidence of obesity?
Is obesity really such a bad thing?
Why is there a social stigma towards obesity?
Should there be a social stigma towards obesity?
If we do think it's a problem that needs a solution, what's the solution?
So my responses:
What's causing the greater incidence of obesity?
Being able to retain fat is a great thing for mammals! It allows them to store energy for when food supplies are low. Unfortunately, in an age of mass caloric availability, this power can cause problems. My understanding of the cause is that people are increasingly leading sedentary lifestyles, spending more hours of the day seated than before; this lower caloric requirement is combined with an increased caloric intake through sugar-saturation of ready meals and drinks. Longer working hours contribute to a rise in easy meals. Unhealthy food can be seen as cheaper (per weight, chips are cheaper than broccoli) though this can be contested.
Is obesity really such a bad thing?
Poor health really is the problem not body weight per se, but carrying extra pounds, and having larger fat deposits puts greater strain on people's system and makes them prone to poorer health. When someone is considered overweight, there's a tendency for people to ascribe health problems they have to the weight when those issues may in fact be unrelated. Also, there are matters of degree here. There's a big gulf between carrying an extra stone or so and being so big you can't leave the house. And being larger doesn't necessarily mean unhealthier (humans are able to store masses of fat for evolutionarily beneficial reasons afterall). Essentially there are two kinds of problems though: problems for the individual (physical activity being harder to achieve, social stigma etc.), and problems externalised onto society (costs to socialised healthcare, costs to provide additional provisions).
Why is there a social stigma towards obesity?
Obesity is seen as a moral failing. This is partly because a svelte ideal body size is pushed heavily and reinforced by culture corps like magazines, Hollywood etc. who are trying to sell things. More than that though, it's seen as a visceral sign of indulgence. People often claim they're worried about the health of the overweight, but this is just rationalisation of the existing prejudice: most of these same people won't be worried about other people who willingly engage in perceived health risks, like motorcross racers or slim people that eat junk food.
Should there be a social stigma towards obesity?
Is it acceptable to be prejudiced against the overweight? On the one hand you have people who think we shouldn't encourage unhealthy lifestyles, on the other hand we have fat activists who say being Fatist is equivalent to being racist. As recent research indicates, making people feel bad about their weight isn't very effective at helping them lose it. If people don't feel they can exercise in shared spaces without ridicule, they're less likely to do so. Something can be a discouraged lifestyle choice without castigating and shaming those that take part in it. Most nonsmokers manage to take up this kind of position with smokers: they'll tolerate smokers without berating them or constantly reminding them of the health hazards (because, of course, these people already know). Still, it's not the same class of prejudice as racism: to be racist is to be prejudiced against someone for something they have no power over, while an adult's weight is arguably something they have in their power to change. In any case, comparisons to different kinds of prejudice are besides the point, when outright prejudice isn't helpful either way.
If we do think it's a problem that needs a solution, what's the solution?
It's a structural issue that should have structural solutions. Education in schools over budgeting and learning easy healthy meals might see more poorer households realising that buying healthily doesn't need to be too time consuming or expensive. As robots take more jobs and we move further towards the post-employment leisure society that the immense wealth of the West can technically allow, everyone will have more time for physical activity and cooking. Until then, we can offer support through socialised healthcare for those that want it, spread knowledge and awareness about the facts, and hold off on the fat jokes, and you know, realise that there's no good reason to get upset about other people's appearances.