He also compares the gospels to Greco-Roman biographies of the time. All (or, at least most- I forget) of the gospels fall within the typical word amount used by these biographers.
I suspect that would probably have something to do with the practical consideration of how long a scroll could be made, without getting too heavy to handle.
Similarly, like the biographers, the gospel authors were focused on Jesus’ public life and death, rather than His childhood, providing only small details about the latter.
I’ve never read any Greek or Roman biographies, but one thing that strikes me about all four Gospels is the focus on the Passion, Crucifixion, and Ressurection. The Passion narrative, beginning with the entry into Jerusalem on Palm Sunday, starts at Matt 21:1, Mark 11:1, Luke 19:28, and John 12:12. A very rough calculation, based solely on the number of chapters, shows that the Passion narrative, including the burial and the Resurrection, accounts for 30 percent of the full extent of the four Gospels, or 26 chapters out of 86.
Breakdown by chapters
Before/after Palm Sunday
Gospel …. Before …… After …… Total
Matt ………. 20 ………… 8 ………. 28
Mark ……… 10 ………… 6 ………. 16
Luke ……… 18½ ……… 5½ … 24
John ………11½ ……… 6½ ……… 18
Total ………. 60 ………. 26 ….…… 86
A more accurate calculation, based on word count or column inches, would probably yield a slightly higher proportion, I think, possibly as much as one-third in some translations.