Link to the question: Why do manga characters love to eat a lot of food?
If the anime/manga takes place in a fantasy world with the term 'unrealistic' meaning something not compatible with reality, something extraordinary-- then yes.
The examples used cite two popular anime characters: Goku from Dragonball (1984) and Luffy from One Piece (1997). Other heroes are mentioned in the comments too, which are all from the sci-fi/fantasy genre.
It is mentioned in the comments, this kind of behavior also exists in non sci-fi/fantasy based anime/manga. It is probably true, I'm sure somewhere, a realistic story of some sort depicts a protagonist stuffing his/her face with so much food-- probably for comedic purpose.
The kind of extraordinary amounts of food being eaten is 'unrealistic'.
But that doesn't mean that such questions on the historic or social context of such themes found in manga/anime shouldn't belong on a sci-fi/fantasy site.
Either from the anime/manga or where it got it's influence from, has found it's way into reality. Look at Takeru Kobayashi and others in competitive eating. That seems pretty unbelievable and unrealistic, yet it's happening in real life. There are extraordinary amounts of food being consumed.
The X-Men also seem to be 'unrealistic'.
Yet, are there not real mutants in the world that showcase some kind of genetic mutation-- say extra working limbs (example: Lakshmi Tatma), gorilla-like hair all over the body (hypertrichosis), superior strength (Myostatin-related muscle hypertrophy), or even long life?
Is life imitating art, or the art imitating life?
These are some examples of stories that are exaggerated that form the basis for sci-fi/fantasy. The question seeks for the historic and/or cultural references that lead to such a theme for such modern Japanese sci-fi/fantasy heroes.
I just wanted to add that the SF&F Stack Exchange community doesn't really have much anime/manga fans asking/answering questions. Whether Japanese cartoons seems on/off topic or not, this doesn't seem to be a good place for such topics.
A simple search (as of 11/8/2012) reveals how many hits are for anime/manga keywords, which is very little. Western sci-fi/fantasy is quite dominant.
Dragonball - 6 hits
Anime - 26 hits
Manga - 21 hits
Star Wars - 626 hits
Marvel - 324 hits
DC Comics - 219 hits
Harry Potter - 811 hits
Dr. Who - 3,301 hits