It is no accident that Samuel, Kings and Chronicles are the three longest books (by word count) in the Hebrew Bible (between 24,000 and 26,000 words each) and therefore the most likely to be divided. The division may have occurred when the Greek translation of these books was made, because the translations are longer than the originals and we can speculate that the books had reached their practical limit in terms of scroll length. Thus anything longer than that would literally be divided.