There is a significant social factor in obesity. Society is set up in a way that encourages obesity in some respects, while at the same time damning those who are not of an ideal body type- this does not bode well generally when people are told that their natural urges are sinful. What we put on the table to eat is heavily culturally influenced, and people in developed countries (especially America) simply do not eat well. While you could look at this ever-increasing evidence to suggest that society pulls the strings as to how an individual manages their diet, there's still agency to the individual, despite environmental factors. Simply put, living in such a society means that the individual is fighting an uphill battle against obesity.
In my mind then, there are similarities between "sizism" and racism in that they are identities society will recognize and treat you differently for. Not only that, but society builds and maintains these identities, and greatly encourages people to not have such identities. Most often being obese can be changed by the individual, unlike racism, which puts the strain on them more because society also places heavy significance on merit; obese people are seen as failures in this regard.
Obesity is clearly not healthy, and society is not seeking to help these people. By placing judgment and blame on them, some of the obese may become spiteful and disdainful at anyone who tells them that obesity is bad for them, and that they should help themselves. As a society we should change our attitudes towards the obese, because at the moment we are, in a sense, simply enabling the problem. Social attitudes towards body image and diet could clearly use overhauling, as well as providing the obese and those at risk to obesity the tools and help that they require to make appropriate choices.