New authors have fairly weak power. Editors can exert their power to:

1. streamline the story such that it flows more smoothly
2. cut out dense detailed descriptions that take the reader from the story
3. their experience at what it takes to make a book become a bestseller gives them authority

The trick is when the author becomes a proven bestseller, the editor becomes weaker. You can look at the books in a series like Game of Thrones and see how the editor became weaker as the book lengths ballooned.

1. A Game of Thrones 726 pages
2. A Clash of Kings  761 pages
3. A Storm of Swords  973 pages
4. A Feast for Crows + A Dance with Dragons 1761 pages

Tom Clancy follows this model as well.