I've noticed that some authors who write perfectly competent novels have trouble in the short story format, often for the very reason you mention - the short stories seem moralistic in a way that their novels don't, perhaps because the novels give them more space to work out their moral ideas? And when they try to cram the same amount of moral thought into a short story, the whole thing becomes top heavy.

Of course, now that I've written this, I can't think of an example author off the top of my head. But I know I have seen it elsewhere.