Hi Thinlynn,
It is not unusual to have one arm be bigger than the other - it is likely the one you use more often. Unfortunately, we cannot "spot train" any areas of our bodies to lose weight in that specific area. There are three factors that control weight loss and it happens throughout the body: diet, exercise and rest. The best way to lose weight and build lean muscle mass (anywhere on your body) is through regular cardio sessions and strength training work.
Walking is one way to get your cardio in. Swimming and aquatic exercises are also great in the summer months, so if you can get to a pool, go for it. In addition to swimming, jogging, intervals, cycling, aerobics, etc. are all terrific. You want to fit in at least three 30 to 60 minute sessions per week getting your heart rate elevated the whole time.
In addition to cardio, you can perform the strength training exercises recommended by alli to build lean muscle mass, which burns more calories and fat, even at rest. These should be done twice per week. Be sure to perform all of the upper body and core exercises, as well as your lower body.
For that stubborn area on your arms, the best exercises are pushups, triceps pull-downs, bicep curls, shoulder press will all help a great deal to strengthen the muscle. Simultaneously, as you lose fat in that area, you will see results.
I hope this helps.
Keep at it.
Good Luck,
Paul
Good Luck,
Paul