Well the Hobbit (and just the material actually in the Hobbit) was always going to be expanded. For two real reasons.
1. Some material that flies by, will actually take a lot of time (the Hobbit is a fast read, because the story is told very quickly). I mean in Two Towers the Battle of Helm's Deep is one regular sized chapter, in a film form its what 30 minutes,
2. They have said that to have a main cast of 13 dwarves, they were going to have to work to make those characters three dimensional. PArt of that is in design, but part will also have to be on the pages, which aren't there in the book.
Then you have the appendix material, some of which is literally just brief notations.
Clearly that material could consume a lot of time, yet only be a few brief statements in appendix form.
Remember the appendix is partial, notes for Tolkien when he was going to fully expand the tale of the Hobbit to fit it into the Lord of the Rings universe, since the Hobbit was create long before all the events and back stories were created for LoTR.