Now I am unashamedly biased here, given I write flash fiction, but much as I love the novel for its wider range of characters, subplots and depth, the short story form has merits the novel can't match. (In fairness not intended to match either). Sometimes less is more and I've come across some wonderful short stories and flash fiction pieces where I think not another word could have been added to that tale without spoiling it. (With great novels, I often wonder about the lives of the characters after the story has finished so with that, there is the potential to add many more words to the tale! It's the opposite reaction to the one I experience after reading a really good short story or flash piece).
This is where I find the 100 words flash fiction challenges really useful and I often come up with what I hope will be a "killer" last line and work backwards to get to that point. If you ever want to learn to write "tight", try writing flash fiction as every word has to count. My bugbears have been using the words "very" and "that" and flash fiction writing has forced me to slash these as it shows up that these are wasted words a lot of the time. They don't contribute anything to the story. The great thing here is learning to write tight like this can then be applied to whatever form of writing you do.