Perhaps its related to the complexity of depression in contrast to the relatively simple nature of obesity as conditions. Depressed people are generally all different, the reasons for their depression may be fuelled by their own social environment or specific situation. The complexity of the issue is deep and broad, meaning it makes it all the more difficult to solve. In contrast, obesity may still be caused by social conditions, but it is much easier to control and solve in the sense that we know that if you stop eating you will not get bigger. With depression, it is very difficult to understand exactly what to do to stop the problem. This may mean that because people lack sympathy for obese people because, in their eyes, it is their problem that they have brought upon themselves. With depression, it is shrouded in taboo and it is more difficult to judge somebody who is depressed because we don't know about their individual circumstances. It's all a matter of assumption and audience.