Obesity causes disability.  Your bad knees/joints/back are either entirely attributed to or exacerbated by obesity.  Same with cardiovascular disease, diabetes, cancer and all the other diseases proven to be correlated to obesity.  I don't think that just being obese is a disability because obesity is entirely reversible whereas true disability is usually not.  Of course, once you've broken your body you'll still be disabled after losing weight, but then I would class you as being truly disabled. Any disability a person has is only compounded by other health problems, so becoming a healthy weight should always be one of the first steps to addressing any health issues.