I mean, Haiku < Sonnet < Opus < Fable/Mythos makes just as much sense. They're larger and larger works.

Many people would get confused on the order of Opus, Fable and Mythos. In my mind, they are even from different groups; fable is a synonym forba fairytale (perhaps with songs), mythos is communicating importance and status instead of a size, while opus is the only one which in my mind communicates a big size. "What did you think about the Tolstoy's book? It was not a book, but a real opus, a tedious, incomprehensible, sluggish monumental opus."

Waiting on Limerick to drop.