Although I'm very lazy when it comes to writing, I'm not that lazy when it comes to thinking. I like to develop the plan of a short story, then cut it as short as possible, try to evolve all the necessary details. I know far more about the characters than what actually comes out of the writing.
As to my writing short pieces, there are two reasons I can give you. The first is my invincible laziness. The second is that I've always been fond of short stories, and it always took me some trouble to get through a novel.