Obesity is a major problem in today's society. The fact that obese people are being more and more accepted to society doesn't mean that there isn't something wrong with it. Obesity causes other diseases but it doesn't make it less of a disease (e.G. MS causes paralysis(a disease) but is still a disease).
Obesity is a mental disorder unless it is caused by diabetes. A few extra kg is healthy because it keeps heat in the body and can also be used when starving, but when you see someone with more than 100 kg (I'm not from the US and obese people have more than by double weight) then you know those lives are destined to a lot of other diseases, higher blood pressure, less mobility, more injuries etc.