One of the biggest traps in topic-based writing it the attempt to size topics so that each one meets exactly one user information need. It is tempting to suppose that this is the point of topic-based authoring. If the book is the wrong size because people only use them to look up bits of information, rather than reading them through, isn’t the point of writing topics to make the topic contain just the information the reader needs in the moment (so that they will read it through)? Thus we envision our reader’s needs mapping to our topic sets like this:
Fewer people read longer topics. But it’s not something to lose sleep over, and certainly not something to shorten topics over.
Tom Johnson has a recent series of posts on topic length (Does DITA Encourage Authors to Fragment Information into a Million Little Pieces?, Do Short Topics Make Information More Findable?, and Why Long Topics Are Better for the User). The discussion around these posts dwells, as all such discussions seem to do, on the question of whether fewer people will read longer posts/topics/articles/etc.