This interview was originally in the French movie magazine "POSITIF" (April 2002 volume). It was translated into Japanese by a Japanese blogger, and I've translated relevant parts into English. Some things may have been lost in the double translation, but hopefully the main points are the same.
Interviewer: By the way, where did Kaonashi, the creature who swallows everything, come from? Also, I noticed in this movie that Kaonashi, Boh, and Chihiro's parents are all obsessed with overeating.
Miyazaki Hayao: That's true. These characters' personalities are full of faults. I made this movie for my friend's two daughters. Like Chihiro, they are also 10 years old. I didn't want to show them something like "the struggle between good and evil." I wanted to show them the truth about the world. If young girls don't really see "good and evil" as something as simple as dualism, then it's something they must discover for themselves in the world. Regarding the overeating, in the past I saw "Babette's Feast". It's a very beautiful and enjoyable movie. In that movie, the characters also eat a lot.
Even when directly asked, he didn't give a very specific answer it seems. Though I suppose Babette's Feast was an important inspiration.
"Kaonashi is inside of everyone." Those are Miyazaki Hayao's own words. Kaonashi can't buy people's attention with money. In addition, he doesn't know how to hold on to people's hearts. Kaonashi is irritated by Chihiro's lack of desires and tells her to want. This is also a necessity of capitalism. There is a contrast those who swarm around Kaonashi when he gives out money and Chihiro's enlightened lack of desire for gold or food. Her resolution in this is so strong that it can even seem cold. There was no reasoning behind saving Haku. He clearly says to Kaonashi, "I won't give you what I want."