I use Dragon Naturally Speaking and use it to get about half to three-quarters of my first drafts down by voice. And I find that I tend to add a lot during my rewrites. My use of DNS tends to be somewhat factual, meaning a lot of the time it reads like, "he said, she said, they went there, they found this, all hell broke loose" and so on.
It's in the rewrites that the story starts to become readable and at the same time, starts to expand. "They went there" suddenly expands to a couple hundred words, starting off something like, "As they raced through the night, the darkness stretched out in front of them. The headlights of the Land Rover tried to pierce the gloom, but it seemed to be threatening to swallow them whole."