The emotional impact of a story is almost always inversely proportional to the size of the story.
It might seem counter-intuitive. Big books, with lots of characters, settings and set pieces engage the reader on a more intellectual level. Worldbuilding becomes predominant–the reader’s enjoyment of the story is more about being immersed in a place and a time. But this always comes at a remove: the reader is constantly reminded of the activity of reading, of the experience of hearing a story. The characters become functions and parts of that world, and the reader is distanced from them, less likely to identify directly with them, for better or worse.
A small story, though, that holds to the Aristotelian unities–one character, events that take place over one day in one setting–forces the reader out of this objective mindset. They have no choice but to relate and identify with the singular voice and viewpoint. Events become enlarged in significance and charge: we know that, since we are alone with the character, everything that happens to them is important, is meaningful in a way we feel more intensely. Our experience becomes more subjective, more immediate. More emotionally resonant.
There are, of course, ways to problematize the difference. Books like Game of Thrones work great at playing with the intersection of these two levels–while there is a huge world out there, with a story rich in incident, each character is given a deep interiority. As a result readers can choose which character they want to identify with the most–who they “root” for. It’s one of the things that makes the book so powerful.