I hear this all of the time from authors who pitch to me. They present a book that, simply based on page count, would fall into the category of many of the series books such as Harlequin. Now, for some of these authors, they simply ran out of things to say and really didn't have a sense of both word count requirements for publishing house lines as well as the difference between single title and series. For other authors, they simply write short books and then just plan to add things to the book to meet the needs of a given publisher. Unfortunately, this is far from the right approach.
A common myth authors have is that the only difference between single title and series (or category) books is the length of the story, in other words, the word count. Although word count does play a factor in all of this, the length of the story doesn't dictate where the story will go. Authors can have 110,000 word series/category books and some can have poorly developed 45,000 word single titles. The difference is really the voice of the story.
When it comes to series and category books, the focus is really on the relationship building and the development of the two main characters. Authors can have pretty complicated story lines, but the focus is entirely on the couples and the relationship. Another way of thinking about this is an analogy I like to use regarding the number of cameras being used to film the story. For most of the series books, think of the focus as being a single camera focus. Because we are using a single camera, the backdrop of the story, the world building (in many cases) and the external things going on in the story simply go away. Readers know the information is there and the authors can tap into the images when they want to, but to maintain the focus on the characters, the author has to decide to eliminate a few things.
When it comes to your single title books, we can often think of the story as using more than one camera, and even lenses that bring in a wider scope of things. We can now see the things going on behind the characters. The scenery, secondary characters and so forth now take on a life of their own and become living, breathing beings in the story. Again, you can have complicated story lines or simple ones, it really doesn't matter. The key is how deep of a storyline you create.
But what does the page count have to do with all of this? Simple. When you are writing the smaller books, you have to pack in a pile of story in a little bit of space. In this case, think of poetry. I can tell a full story in narration, but once I limit myself to the line length, rhyme structure and so forth of poetry, as an author, I have to include what is important, make every word counts and really create focus. When we have a single title story, we can include things that we might not normally include, and therefore, the word count increases.
As I pointed out though, it is the voice and the focus of the story. This is where authors make a mistake by simply "adding" to the story. Just putting in more scenes or more words that have a category voice to it becomes repetitive and often times episodic. Stories drag and readers start to question why the character didn't learn something the first time and they have to make the same mistake 2, 3 or 4 times.
I have said this before and I will repeat this again. Authors have to decide where they want to write and then start working on the story with that goal in mind. You have to know the voice of the publisher and certainly know and understand your writing. Look, writing category or writing single title is just as rewarding. One is not better than the other. Just remember that you cannot simply make one style of writing fit another.