It's not a disease, afaic, but its not the same for everyone. I'm pretty overweight. I have friends who are way lighter than me who exercise less, eat more shit, and sit around more. So while, yeah, it's my decision to not put in the hours to get as light as them, it's also a little more complex than a fat person being lazier than a skinny person.
"Let's all get the same girl pregnant, tonight!"