Research indicates the rate of diagnosed arthritis in obese people are double the rate of a person with normal weight. For every 1 average weight person with arthritis, there will be 2 obese persons with arthritis.
http://arthritis.webmd.com/news/20101007/arthritis-on-the-increase-obesity-partly-to-blame
Common sense tells us that overweight people tend to move around less, and this will likely lead to stiffen joints due to lack of activity. Or when you are so heavy, it can lead to overexertion of knee joints and develop into knee osteoarthritis.