Once in a while I come across a novel that should have been a short story or a short story that should have been a novel. What seems to be the problem is:

A short story that should have been a long story =
a character-focused story that doesn’t pan out because, like an awkward date between two people that don’t know anything about each other, the character hasn’t fed the reader enough reasons to care

A long story that should have been a short story =
a plot-focused story that hangs on the empty framework of shallow characters and drags with a Monty Python-style “Get on with it” chorus chiming in every chapter

Although there are examples of short stories with a rich character (note the singular) and plenty of examples of novels that keep the pages turning on the power of their world or story alone, they are the rare case. In general, it’s safer to decide:
“Is this about how a character adapts to challenges and grows?” = Long Story
“Is this about a cool idea I want to explore?” = Short Story