The director and actor are among members of the cast and crew who have both spoken out against claims that turning the originally planned two-film adaptation of the short novel into three movies was a cynical, money-making ploy.
"The book is written in a very brisk pace, so pretty major events in the story are covered in only two or three pages," Jackson said, according to The Hollywood Reporter.
"So once you start to develop the scenes and plus you wanted to do a little bit more character development, plus the fact that we could also adapt the appendices of Return of the King, which is 100-odd pages of material that sort of takes place around the time of The Hobbit, so we wanted to expand the story of The Hobbit a little bit more, as did Tolkien himself.
"So all those factors combined gave us the material to do it."
Co-writer Philippa Boyens added that, having filmed The Lord of the Rings, they put in extra material which they might otherwise have left out.
Richard Armitage - who plays Thorin Oakenshield in the upcoming trilogy - said that the film takes time to give separate personalities the company of 13 dwarves, which were not detailed in the book.
[Left: Jackson on set / Right: McKellen as Gandalf]
"Condensing it into two films seems almost impossible," he said.
"Anyone who thinks Peter Jackson would fall for market forces around him rather than artistic integrity doesn't know the guy or the body of his work," added McKellen.
"If we just made one movie, The Hobbit, the fact is that all the fans, the 8-, 9- and 10-year-old boys, they would watch it 1,000 times. Now, they've got three films they can watch 1,000 times."
The Hobbit: An Unexpected Journey will debut on December 13 in the UK and December 14 in the US.
Gallery - The Hobbit: An Unexpected Journey