Both Tolkien and C. S. Lewis were members of the Scriblings (an Oxford University group of writers) and they were very good friends who critiqued each other's books. But they were opposites, writing-wise. Tolkien held worldbuilding paramount. He did the worldbuilding before writing the stories; it was his hobby. Lewis wrote Narnia on the fly. He had no idea where the Chronicles would go. He started with one book, and built little by little off of that.
The Lion the Witch and the Wardrobe introduced Narnia, but we didn't see much of it. In
Prince Caspian, the reader discovers Narnia is part of a larger world that includes other countries (Telmar and Archenland). In
The Voyage of he Dawn Treader, that world is larger yet, with a whole eastern ocean full of strange islands, a few other countries, and a magical wave into "Heaven." And on and on. Each book built on what came before. By book 7, the world was as rich and full as Tolkien's. But it wasn't planned like Tolkien's. Lewis left himself breathing room between each book. I prefer Lewis's method of creation, myself.
On my blog I wrote about Narnia all last summer.
If you've read all of the Chronicles in-depth, there's A LOT of worldbuilding that doesn't make sense though. The country of Telmar is created and abandoned, and its real location is up in the air. There is no mention of tundra or an arctic for the White Witch's polar bear fur and reindeer to come from. And there's the question of Mrs. Beaver's sewing machine -- if Narnia was a pastoral state, where did it come from? In Tolkien's world, everything was more precise and footnoted exactly.
I love both Tolkien and Lewis very much, but have more personal fondness for Narnia.