A serving size is not a portion size. A serving is the recommended amount of a certain food to eat. A portion is the amount of food we choose to eat. Be very wary of packaged foods. Almost all of them contain more than one serving. Read carefully the nutrition label. And divide the contents accordingly. For example, if your can of black beans contains 3 servings, separate out 3 equal-sized servings. Then, prepare 1 and save the other 2. The link here provides other guidelines for serving sizes.