Your entire response is basically "people are fat because they are bad." Of course, you don't say it that way, but when you say "We're all responsible.." you are both being trit - 1970's pop psycholoy - and remarkably blind. And if you had the empathic ability to understand how others perceive your words...

Since that time of pop psychology, a lot has been learned about genetics - including the fact that body weight is significantly heritable. Epigenetics was discovered (you'll no doubt have to look that up). The complex feedback systems in the body have been determined in more, if not perfect detail. Insulin resistance has become better understood. And, things thought well understood in psychology were found to be nothing but superstition, especially the '70s pop-psychology or even worse, the Freudian and Jungian theories that preceded it.

But to you, it all comes down to "being responsible."

If that were true, than anyone could be a world class athlete, if they just wanted to and were disciplined enough. After all, there couldn't be anything in their body holding them back, right?

Understand: humans are very diverse. Your responses are overly simplistic.