So I would put it in the collection and set it apart as GMW said. Do not, under any circumstances, just pad it for the sake of padding. If it is a short, let it be so.
If you wanted to make it longer could you write more before the current version starts? Meaning - spend more time on setup? I do not know the story, but I bet it has characters in it, and they had to grow up as some point. Is any of that relevant?