It was easy to insert a little extra dialogue here and there and all of those 400 words gave the piece more sparkle. After all, I was already very familiar with my characters - they were like old friends - so it was a way of reacquainting with them and extending previous conversations.
It is also enjoyable to shrink a story. I love winkling out the unnecessary words, or even sentences, and tidying the whole thing up. It feels neat and polished. It is also satisfying to check the word count and discover that you have reached the desired total.
Occasionally, I have increased a rejected story, had it turned down again, decreased it for a new market, had another rejection, then re-increased it for a third try!
Never be afarid of stretching or shrinking your fiction. It is a living, breathing, flexible creation. It is not set in stone.