Assumptions
Matthew and John, eyewitnesses to Jesus' entire ministry (John 15:27), were promised they would remember everything that he taught (John 14:26, 16:4). Luke (and to a lesser extent Mark), however, were not eyewitnesses of Christ's ministry. It would therefore be easy to assume that Matthew and John would have the biggest gospel accounts, but this is not the case!
The Largest Book
The gospel of Luke, in the King James Bible, has 1,151 verses and 25,952 words. It has more verses and words than either Matthew (1,071 verses, 23,727 words) or John (879 verses, 19,123 words). It is also comparatively huge when compared to the book of Mark! Luke has 70% more verses (1,151 versus 678) and 71% more words (25,952 versus 15,177) than Mark.
Luke is not only the largest gospel, it also is the biggest book in the entirety of the New Testament. It is a staggering 88 times larger than 3John, the smallest, with its 294 verses. Even its first chapter, with 80 verses, is the largest in the New Testament.
Below are three of the main reasons why Luke is the biggest of the gospel accounts.
Sources of Information
Matthew and John based their gospels on the teachings and events they saw and remembered (see Matthew 23:31, Acts 2:32, 5:32). Mark relied on Peter's recollections for his book. Luke, however, could not write about what he heard and saw since he was not among Jesus' original disciples.
Luke, unlike Matthew and John, compiled his book not only from existing records, but also from the plethora of witnesses who were still alive (see 1Corinthians 15:6). This abundance of information led him to create the biggest gospel. He did this research when Paul was in a Caesarea prison (Acts 24).
"In his opening statement, Luke mentions briefly how he prepared his Gospel. Since he himself had never seen or heard Jesus, he obtained the material for his book from careful research of existing records and from the accounts of eye-witnesses" (AMG Concise Bible Dictionary).
"During Paul's Caesarean protective custody, Luke had free access to Paul. It was during this time that Luke must have written his Gospel account . . .
"Jerusalem was not far from Caesarea, and it would have been easy for Luke to go to Jerusalem, where the eyewitness records of Jesus' ministry must have been kept." (Holy Bible, a Faithful Version, second edition, page 51).
Luke interviewed the original apostles still in Jerusalem as well as Jesus' mother Mary and many others. She would have relayed information only she knew which she kept in her heart (Luke 2:19). This included the events surrounding John the Baptist's birth, the birth of Jesus rejoiced over by angels, the Lord's dedication at the temple and more.
Luke 1:5 to 2:38 uniquely conveys its events from Mary's point of view (International Standard Bible Encyclopedia). It therefore records details and facts not found in the other three gospels.
In-depth Research
Luke's research also turned up other events only briefly, if not at all, mentioned in Matthew, Mark or John.
Luke 9:51 - 56 discusses Jesus and the disciples traveling south through Samaria starting in mid-November of 29 A.D. He then delineates what took place from this period to when the Pharisees warned Christ that Herod wanted him dead (13:35) in late December of 29. Matthew and Mark's gospels only briefly reference this journey (Matthew 19:1 - 2, Mark 10:1).
Luke is additionally the only one of the four gospels to list the events from Jesus' healing of a man with dropsy (Luke 14:1) to the Lord giving the parable of the Pharisee and the tax collector (18:9 - 14)! This covers the period from approximately January of 30 A.D. to sometime in February just before Lazarus is raised from the dead.
Goals and Audience
The second and third reasons why Luke's book is the biggest revolves around goals and audiences.
Matthew's book, written in 35 A.D., was the first of the four gospels to be completed. The audience he targeted was Jewish. He therefore strongly emphasized Jesus' role as the Messiah who fulfilled Old Testament prophecies.
Mark's gospel aimed at a Gentile audience. His goal was to put emphasis on Jesus as a miracle worker and someone who overcame Satan the devil, sin and death.
John's gospel, finalized around 95 A.D., was the last of the four to be completed. His primary audience were converted Christians who wished to learn more about the Lord. He therefore focused his writing on Jesus as both man and God. He also sought to reveal his profound teachings reserved for his closest disciples (John 12:1 to 21:23).
The third gospel, of all the accounts, had the broadest goals and audience in mind.
"He wanted to confirm for Theophilus the certainty of the things Theophilus had been taught. Luke also wanted this information available for a wider readership.
"Most scholars conclude that Luke's target audience were Gentile inquirers and Christians who needed strengthening in the faith." (Holman Bible Dictionary).
Luke's book gives a greater overall coverage of Jesus' life than the other Gospels (AMG Concise Bible Dictionary).
Conclusion
Luke, unlike Matthew, Mark and John, was not Jewish. He was an outsider, a Gentile (Greek) who converted to Christianity more than a decade after God's church started. God still used him, however, to produce a gospel more orderly, detailed and thorough than the three others. The depth of written and personal eyewitness accounts Luke researched, along with the goals and audience he sought to reach, led him to produce the biggest gospel!