my /usr/share/doc/gimp-user-manual-2.0 directory is 31 MB big. This should be optional, controlled by the doc USE flag.
If you are installing app-doc/gimp-user-manual, what would it install _without_ USE="doc"? The whole point of it is splitting the documentation out of the media-gfx/gimp install. If you don't want the user-manual, then emerge -C gimp-user-manual, or don't emerge it in the first place.