Well, story ratio is all about how the various components of a story are distributed. For instance, the ratio of description to events, or the ratio of subplots to the main plot. A good story ratio makes the narrative flow smoothly and keeps the readers or viewers interested.
The concept of story ratio means how much of one aspect of a story compared to another. Say, the ratio of romance to adventure. It helps determine the genre focus and how well the story elements complement each other to create a coherent and enjoyable tale.