I'm about 230 lbs and have never had any problems fitting on anything.
My mother, is a larger lady than I (I couldn't tell you her exact weight, being that I don't know it, and it would be rude anyway) and the only thing I ever recall her flat out -not- being able to get on is Dr. Doom's Fearfall. She's typically gotten into Dueling Dragons and The Hulk both fine using rows 3 and 6. She does occasionally have to squeeze a bit more with The Mummy, but gets in. I never recall her having trouble with anything else.
As others have said, I think alot of it also depends on how your weight is distributed. Your build, as it were.