"Short on space" means that there is not any room for an object. Example: if you have an apartment that is very small and you had a piece of furniture that you like ( couch, chair etc), but that item will not fit in your apartment because that apartment is too small . You would say "I like that chair, but I'm short on space so I can' t buy it". This means that the chair just won't fit in your apartment because the apartment is so small. I can't think that this would ever apply to earbuds since they are so small..