The Dalek Invasion of Time is also a tutorial. The first dozen levels are meant to introduce the core mechanics required to play Doctor Who Infinity. It's a free area we have created to introduce the game and when the game goes live on mobile, this will be included as the content you can play with the free download of the application.
Hence it had to be much longer by necessity.
The Orphans of the Polyoptra and The Lady of the Lake are good indications of the length you can expect from subsequent releases.
Please note though, when we create these stories, we don't specify a word count from the authors. This is there sandbox to play in and therefore, by its nature, there will be variety in length depending on the story they chose to tell.