A short story can be of better quality because it requires precise language and a tight plot. With fewer words, the author has to be really sharp in creating characters and delivering the message.
Short stories are often better quality because they force the writer to be economical and get to the heart of the matter quickly. There's no room for fluff, making the story more intense and engaging.
Well, short stories tend to be more focused and concise. They don't have the space to meander, so every element is carefully crafted for maximum impact.