If you are ridiculed doing something actually ridiculous, especially something that's your own fault, you're going to have a much harder time defending it...
Sure, there can be situations out of your control - I, for example, do believe that my genes aren't helpful to my weight based on family members and anecdotal evidence, but at the end of the day if I really cared I'm perfectly aware that I could be skinny, I just happen to chose to eat great food, and too much of it, and do far too little exercise.
If you mock me for being gay I might be offended, but for being fat.. that's my own damn fault, I know how to stop being fat but I don't care enough to do it, currently. If anyone wants to laugh at that, give a shit,
1) It's a lot easier to get out of shape and overweight, than it is to get back into shape.
2) Being overweight can cause emotional problems that make it even harder lose weight. Laughing at someone worsens these problems.
Having said this, I guess it's human nature. I find myself at times making fun of people (in my head) for how they dress or look. At least I'm trying to overcome that kind of thing, though. Doesn't seem like most people even recognize shortcomings/try to get better.
People tend not to argue.