The general thinking about fat people is that they got that way because they were too lazy to move. I don't think that we can blame The Simpsons depiction of Homer as someone who spends his free time lazing on the sofa watching T.V. eating pork rinds or chips, but it sure isn't helping.
I can't speak for everyone who is overweight, but I rarely watch T.V., let alone do so while snacking on salty treats. Personally, I've known far more thin people who act like couch potatoes than fat ones. In terms of sloth, I have never witnessed a clear difference between people based on body weight. In fact, my husband is unabashedly a lazy person, and I rarely rest. That doesn't mean my activity is aerobic, but I'm always working on something. How can I be "lazy" if I never stop working?
The difference tends to come in when one looks at structured exercise. A lot of overweight people don't go out and jog, attend gyms, etc. Many people believe this is an indication of innate laziness and a lack of concern for one's well-being. That's a pat judgment which I believe rarely reflects reality. Most people who are overweight and sit around snacking at night have boring jobs that wear them out, kids to look after who tax their free time and energy, or, and this has been an issue for me, have a fear of going out in front of others and moving.
I've wanted for quite some time to just go out and walk for as long as I can everyday as a form of exercise which I can manage at this stage in my weight loss plan, but I had to work up a lot of courage to do so. If you can't guess why, then I'll tell you a little story from my youth that sums it up pretty well. One of my friends was a year or so younger than I and reported a situation that occurred in her class one day. They were in English class and my gym class was playing softball outside their window. When I came up to bat, the kids in her class had a high time making fun of me and how I was fat, slow, and not very good at playing. To his credit, her teacher defended me as someone who was smarter and harder studying than the lot that were mocking me (bless his heart and soul).
The point here is that any time a fat person tries to exercise in public, he or she has to face public judgment and mockery. Instead of people looking at the fat man out there trying to jog so he can lose weight and improve his health, they're tittering, whispering behind their hands or out and out making fun of him. If you knew that you were going to be belittled and humiliated every time you tried to engage in a physical activity, would you be out there engaging in it? In the end, one can exercise at home, but that requires a different sort of motivation. It isn't as healthy or enervating to jump on an exercise bike or elliptical trainer in your basement. Leaving the house is a cue for movement and gets you out of the environment which you are usually distracted in. Staying in it is a situation which requires you to cope with whatever else may interrupt your exercise like other people's demands, the phone, the doorbell, or nagging notions of what needs to be done around the house.
Fat people are not slothful. They work plenty and move plenty. If you don't see them exercising, then it probably has to do with society's attitudes toward watching them move. It's hard enough when your body is large and ungainly to exercise without the added notion that you're some sort of comedy floor show for the world.