I'm a fairly large person myself; was once nearly 290 lbs, down to 268 currently. I've had problems on rides before, but only once have I been denied a ride, and that was due to my friend sitting on my seat belt and not realizing it. The most embarrassing moment though was when once when I got on Thunderhead. It seemed that the belts were shorter in the front row than on the rest of the train. The belt wouldn't fit around me easily, and the ride op instructed me to lift my belly. People need to realize that if they are fat, they may not be able to do some things that most other people can do.
I think the limiting size of the restraints unintentionally serves a dual purpose. Heavier people tend to be unhealthy and have more heart problems. The ride could aggravate a heart condition and the person will return to the station no longer among the living. Denying them the ride could save their life and open up their eyes that they need to make a lifestyle change.