Nutrition and cardiovascular exercises are necessary to reduce the size of your belly. Ab exercise won't reduce girth. They will strengthen your core, but you will need to lose excess body weight. If you want large abs, with deep valleys, as you would see in a body builder, you would need to do the following exercises once every 5 to 7 days, allowing your abs to have time to recuperate. Begin with crunches, with weights, doing 2 sets of 8 reps. Then, you will need to do bicycle crunches without weight. Then, repeat. Hold the weights behind your head. Start with your maximum weight. Then, reduce it in half. Then, do bicycle crunches with weights. Then, repeat the series without weights. Next, hold the side plank pose on each side.