> an Opus is longer than a Sonnet

And people know this? I didn't. I am not into music or poetry so these are not terms I am familiar with.

An opus is just short for "magnum opus", and it's a different type of label than a sonnet. A sonnet is a very particular kind of poem, while "opus" basically just means an important work. It can be short or long, and has no format requirements like sonnet (or haiku does).

And fables are not particularly long actually.