I think it depends on the exercise, especially with a multi-varied activity like CrossFit. On the rower, a fat person will be fighting their belly as they stroke. A heavier person is going to have a harder time with pullups or muscleups. A heavier person is going to have more joint impact during running or some of the Olympic lifting movements. An obese heavy person (as opposed to a low BF heavy person) is likely going to have less cardio ability.