The amount or portion size is a different measurement, not meant to be indicated by calories, but intended to give an idea of what a normal portion size is to give people an idea of what a normal portion looks like i.e. a 40oz steak is not a single portion.    

This is to help members understand that the huge portions commonly served in restaurants are not normal or healthy portions. Nonetheless, they are guidelines only. Someone could have more or less based on how important the food is (food ranking), how often they have it (special occasion vs. daily), their caloric intake (it could be more for bodybuilders or pregnant women, for instance, less if less active), etc.