When I was in high school, I read a story about a new mother whose husband had died suddenly, leaving her with an infant. She went about her business during the day, but at night she wept over her infant in his cradle. She did this night after night, eventually transferring her sorrow to him.
I can't remember the name of it or the author, or even how it ended; but 30 years later, I still remember the impact it had on me.
Edgar Allen Poe was a brilliant writer of short stories. His thrift with words left nothing undone for the reader, but served to enhance the reading experience because of the brevity of the story.
The point is that an author does not have to write a tome to have an impact on his reader. Read "The Lottery" by Shirley Jackson or "The Yellow Wallpaper" by Charlotte Perkins Gilman. These stories impacted publications, society and changed the author's lives. You can feel the emotion in them, smell the air and see the walls move. The ability to enrapture a reader in less than 10,000 words. A complete mental vacation in, as Poe put forth, a single sitting.
What is a short story that had an impact on you?