its an interesting observation, but I wonder if it is really connected to the intensity of the class. It could be a lot of things- are the classes at different times? For example, if the zumba class is during the day, maybe people who are home during the day go to that (and might be heavier - maybe they are home during the day because an injury prevents them from working AND being active). Maybe some of those women prefer dancing to other workouts because they are bigger and feel less intimidated by it. Or maybe it kinda just started with a group that were bigger, which made bigger women feel more comfortable being there than with the thinner/fitter women, so they take the zumba class instead. I think really you have to judge the intensity for yourself- what's intense for you might not be intense for someone else and vice versa