For many overweight exercisers, every step of a workout comes with an unintended cascade of motion — breasts bounce, belly fat shakes and thighs rub. The added jiggle and friction of moving body fat is more than just bothersome. It can alter people’s gait and make them more prone to injuries and joint problems. The discomfort prevents many overweight people from exercising altogether.I think they're on to something here. This issue is probably so significant that many would rather have a total knee replacement first.
Reference: Browning RC and Kram, R "Effects of Obesity on the Biomechanics of Walking at Different Speeds." Medicine & Science in Sports & Exercise. 39(9):1632-1641, September 2007.