I think more detail must play a role in it; Tolkien may go for more description than Lewis does in Narnia. Also, comparing it with The Hobbit may be more useful? It is, in a way, a different genre.

Narnia and TH were written for younger readers, hence shorter chapters, but yes, I do think detail plays a part in that. With Narnia, Lewis kept the descriptions pretty sparse, allowing readers to envision the story as they saw fit; whereas Tolkien wanted to project his vision of Middle-earth to the readers.

Precisely. Narnia can take on the reader's own experiences (Lewis even kind of leads the readers to do that with some of his descriptions).

Now, that is true.

I would say The Hobbit is intermediate between Narnia and LotR as far as story-telling is concerned. On the other hand, Silmarillion is even more sparse in detail, a chapter is often like the equivalent of a whole Narnia book.

The Silmarillion I wouldn't really use as an example of storytelling in general. Tolkien had massively thought out outlines and his son tried to put them in some sort of order, it's almost more like a history textbook than a story. But that's my opinion 😛

Mine, too. The few finished works covering those times would be a better indicator of how Tolkien might go about it, but I've actually never read those...
On the other hand, even history books tell a story and have ways of going about it, so from that point of view, it's just as interesting to compare, seeing as Lewis never went into that kind of detail.

I remember loving descriptions when I was young. I thought it was so important for an author to describe everything so the reader would get a clear and exact picture in his mind. Narnia was what made me really, truly realize that you don't need all that necessarily, since I just ended up making up my own clear-as-a-bell mental images as I read. (Granted, Lewis did describe some things more than he did others, plus the illustrations were of some help.)

Well, history textbooks are also stories.

Livy is a historian, and we get lots of stories of Roman Antiquities (like Romulus and Remus, or carreer of Numa Pompilius) from him, unless we know Greek as well as Latin.

Do you know where I can find the word counts per Narnia book? I have been wondering that for a while.

I am unsure of how to describe Lewis' style because I compare other books I read to it, but the narrative never confused me.
I feel it is also easier to read Lewis' style than Tolkien's, but Tolkien's style for LOTR was very immersive when I first read it when I was twelve. Maybe I was just the right age, but Middle Earth felt very real because of all the consistent detail Tolkien wrote, and I was glad to see how visually similar the movies looked in the way of color schemes and settings.

Word counts?

Try counting pages, and in every page where there is a picture or a chapter heading or a chapter end, how many lines are left out.

Then count words per line.

For safety, count words per line in normal and in dialogue, and make separate calculations.

I always thought the way Lewis occasionally steps out of the narrator role to address readers directly (e.g., to admonish readers never to close the door and thus shut themselves up in a wardrobe) was a charming effect, making me feel more connected somehow to the story-voice.

I always thought the way Lewis occasionally steps out of the narrator role to address readers directly (e.g., to admonish readers never to close the door and thus shut themselves up in a wardrobe) was a charming effect, making me feel more connected somehow to the story-voice.

It worked well when my mom used to read the books aloud to my sister and I when we were little. My mom, I think, especially loved those parts.