Catsmother - what about the difficulty of exercising ('just moving around more - OP') when you are very overweight?
As I said in an earlier post - I weigh 20 stone - so it is like a person of my ideal weight carrying around another person pretty much the same weight as them all the time. It makes exercise bloody difficult, I promise you. By the time I get to the end of our road, my back is aching, and if I walk a decent distance, I'm crippled afterwards (like when my friend and I walked 4 miles with the dogs, and I could hardly drag myself upstairs that evening).
Another thing - and I know this is going to sound like such a trivial reason - when you are really big, it is very hard to find exercise clothes that fit, that are comfortable to exercise in, and that don't make you feel like the world's worst heifer when you wear them. It took me ages to find yoga trousers that fitted me and that didn't shrink the first time I washed them, and even longer to find a loose t-shirt that was long enough to cover my stomach.
If you feel totally conspicuous already, just by being at the gym with all the fit, healthy, slim people, and you are made to feel worse by the clothes you are wearing, it just adds another factor that can help put you off exercise.