''When we retrieve a memory, we also rewrite it, so that the
  time next we go to remember it, we don#t retrieve the original
  memory but the last one we recollected.

  So, each time we tell a story, we embellish it, while remaining
  genuinely convinced of the honesty of our memories.'' G. Musser,
  Scientific American

  And that's why the fish grows everytime the story is told. They
  don't even know they're embellishing an already embellished
  story because they really, truly, doggone it believe it.