Let me try:
1. Depends on potential size of the composition. More frequently there are minor corrections, but occasionally I need larger ones as well. I am intending to have a larger revision of my new choral piece "Svetla pesem". I am not happy with it at all. :)
2. Mostly the changing things are for easier performance (technical reasons).
3. I am not really sucker for later revisions. Once the composition is performed, it should have remained untouched, unless there are serious problems in it. For example: Sibelius was unhappy with his composition Kullervo and forbade its performance during his living. That was a stupid thing to do since it has turned out to be a very mature and fine composition in his opus. And Rautavaara has three or four versions of his first symphony, with the most used two-movement version being relatively strange with long slow movement and short scherzo finale. I prefer to use the experience in future works while being aware of weaknesses of performed work.
4. My targer is usually to make the form a bit longer - I often make the first version too short. But I don't give the score out until I am satisfied.