Obesity is a risk factor for the development of knee arthritis.
There is a direct relationship between your weight and progression of arthritis on xray.
When you walk, three to six times your body weight goes through your knee joint. If you are 10kg overweight then your knee carries an extra 30-60kg of weight each time you go up or down a step. There are studies that show losing as little as 5% of your body weight can reduce your pain both in the short-term and long-term.
Pain and disability in your knee can make it difficult for you to exercise sufficiently to lose weight. This can be improved by exercising in water to reduce the load through your knees (swimming or hydrotherapy) but ultimately changing what you eat will be necessary. For most people this requires regular sessions with a dietitian who can teach you how to reduce your calorie intake and how to use a food diary to change your eating habits.