Science establishes that it's unhealthy and you should do what you reasonably can to reduce the condition, but many people are medically unable to or live in economic conditions that make it difficult to. Improving labor laws, health care availability, and education for all the people we can would be a great and charitable good.
However, one must also be aware of the inaccuracy in diagnoses of obesity. It is usually diagnosed by BMI measurement, but that has been proved to be unreliable. For more on this fact see the Wikipedia discussion of it and the summary of a recent study on the Accuracy of Body Mass Index in Diagnosing Obesity in the Adult General Population.