The best way to visualize hollowness is a tube. A tube my look thick and heavy from the outside, but all of its material is at the surface.
By contrast, of you took that hollow tube and crushed it into a ball that had no air in it, you would have a significantly smaller object, but it would be far more dense.
A pitfall that writers too often fall into is mistaking word count with quality or depth. If you wrote a 300 page book, and I skip over the equivalent of 50 pages due to wasted words and paragraphs, then you have a hollow story. And hollow stories are weak stories.
With that said, dense stories do not waste words, but they also don't leave any important ones out. If, in trying to condense your story, you cut out so much that your audience misses your message, the. You still end up with a hollow story, just in a different manner.