I’m very late to this thread, but feel compelled to weigh in anyway… 
fluiddruid and Stoid have the right of this, and I urge everyone here to check out the available scientific research on the mechanisms of obesity. There is a reason that most obesity researchers don’t consider overweight to be a character failing. Everyday, they encounter new evidence that weight regulation is a complex biological process more akin to breathing or bladder control than to purely volitional acts.
Yes, it is clear that most fat people are fat because they eat too much. This has never been in dispute. What is not clear is WHY they eat too much. Fortunately, there are scientists attempting to piece this together with careful research rather than idle, “common sense” speculation.
As a quick example… we know that fat cells produce a hormone (leptin) that suppresses appetite. When a normal-weight person overeats enough, he accumulates fat which then pumps out more leptin, which suppresses his appetite, causing him to return to a set weight point. This is an unconscious process, requiring little or no active volition for many people. On any given day, this person may overindulge in a delicious chocolate cake, which he then avoids the next day because the thought of eating more of it makes him feel ill. He may assume that he is exercising self-control here by avoiding the delicious cake, but the truth is that he has a lot of help from his body and doesn’t need nearly the same level of self-control that would be required from a leptin-deficient or leptin-resistant individual.
A small percentage of the obese population has been found to be leptin-deficient. They just aren’t producing enough of this hormone and never feel sated. Given free access to food, they will eat their way to morbid obesity and/or death. Inject them regularly with leptin and they will slim down almost overnight.
A much larger percentage of the obese population has been found to be leptin-resistant. Unlike the deficient folks, they actually have very high levels of leptin (as to be expected with high levels of fat), but they lack sensitivity to it. The effect is very similar to deficiency, but the cure is not as simple. Injecting more leptin may help a little bit, but is kind of like giving salt water to a thirsty man adrift in the ocean. The real problem in this case is faulty or missing receptors – which are much harder to fix. Perhaps gene therapies may one day be available to correct the problem.
Leptin is just one small part of obesity research. There are numerous other promising angles being explored. Thankfully, there are good scientists approaching this as a biological puzzle to be solved, not as a character flaw to be overcome.
Millions of people experience some level of urinary incontinence and find it difficult to hold their bladders for extended periods of time. In cases where no bathrooms are available, some can exercise extreme willpower to overcome this weakness, while others need to wear adult diapers or embarrass themselves by leaving in search of relief. Most of us consider this to be a private medical issue, not a character flaw.
A drowning person often hastens the process by inhaling water. He knows that it will kill him, but at some point, his instincts overcome his willpower and he tries to draw a breath anyway. Most of us do not consider this to be voluntary suicide.
The mechanisms involved in weight regulation play out over much longer time periods (months and years), but seem to be just as regulated by biology as breathing and voiding. Willpower can help you overcome instinct for a while, but unless something substantial changes in your environment, your biology is likely to eventually overcome your volition.
Instead of blaming fat people for their condition, instead of assuming that everyone’s body reacts to food in the same way, and instead of insisting that they fight their biology with every ounce of available willpower, let’s just give credit where credit is due. Give kudos to the person who loses and keeps off 200 pounds just as you’d congratulate the ultra-runner who just finished another 100-mile race. Lay off the 300lb guy who does other things with his time, the same way you’d respect the privacy of the incontinent fellow who needs to wear Depends. And above all, support the scientists who are working to level the playing field so that we can all enjoy the benefits of healthy weight regulation without conscious obsession.