Short stories are usually a subset of a Grand Argument Story. This means that they typically do not go to the depth of a full story, or the breadth (cover all of the throughlines) of a full story. "Epics" usually have one "main" story embellished with LOTS of substories -- stories that are outgrowths of the main story. These substories frequently have one of the objective characters act as the Main Character of the substory. Some "epics" have more than one main story going on. In these cases, it is necessary for the author to be clear about what storytelling belongs to which story. In addition, these stories frequently ALSO have many substories tagging along.