Any number of inflammatory-response diseases tend to be treated with Prednisone or other forms of steroid, which have weight gain as a byproduct.

Also, appearances can be deceiving. Though many don't believe it, it is possible to be quite healthy and active while carrying extra pounds, appearing to all the world to be "lazy, fat slobs" that don't "get off their arse". Sumo wrestlers and football players with large BMIs immediately come to mind (believe it or not, sumo wrestlers are athletes with very little internal body fat), as well as average Joes and Janes who do exercise regularly but don't appear stereotypically "healthy". It always amazes me how people can easily team up to jump on the fat insult bandwagon but if you substitute gay, African-American, or other phrases in the insult, people wouldn't be caught dead saying them because THEN they would not be PC.