Before providing a response to this question, I should acknowledge the depth and breadth of scholarship (controversial and otherwise) associated with this topic—it is indeed vast. While the Q hypothesis (from Quella, meaning “source”) is an interesting idea, bear in mind, it is just that—an hypothesis. Debating the idea is really not too profitable (in my humble opinion) or essential for understanding the Gospel. But I will try to succinctly provide some context (and hopefully some understanding) of the discussion of the “Double-Source” theory.
To start, each gospel account (Matthew, Mark, Luke, and John) arguably has its own theological agenda (as well as sharing common themes) and most likely a specific audience in mind at the time of writing. Or one could say, there is one gospel depicted by four witnesses. Also, Matthew, Mark, and Luke are considered “synoptic” sharing similar material (to oversimplify it), whereas John is “autoptic” being more unique than the previous three gospel accounts. These similarities and differences of the gospel accounts is the catalyst of so much debate and discussion.
Theories as to how the Gospel accounts were formulated and preserved include: 1) Oral Tradition i.e., word of mouth (oldest view); 2) Fragmentary Theory—the sayings and deeds of Jesus were preserved in separate detached form; 3) Interdependence Theory (also called “Reciprocal Borrowing”, “Mutual Use”, and “Immediate Independence”--but there is no agreement on who borrowed from whom (there are essentially six different combinations); 4) Urevangelium Theory (also called “Mediate Independence”)—presupposes the existence of a “primitive Gospel” from which all three Synoptic accounts drew their material (Logia); 5) Documentary Theory (this would include the notion of a ‘Q’ document; 6) Formgeschichte Theory (Form Criticism)—Method of study dealing with the pre-literary stage of the Gospel tradition attempting to determine the influences that acted on the material while it was being transmitted orally. This theory dictates that the evangelists were not the authors, but merely collectors and editors of pre-existing forms.
As for why Mark is brief compared to the other gospels comes back to his theological agenda and perhaps his target audience. Where Matthew was, arguably, writing for a Jewish audience and concerned with illustrating Jesus’ kingship for example, Mark, writing for Roman Christians, might be primary concerned about illustrating Jesus' “servanthood” and "sonship" (bearing in mind both are attributes of Jesus’ messiahship as well).
And, for what it’s worth, there are clear examples of literary and theological intention in all four of the Gospels. Each gospel writer wrote his account to emphasize certain aspects of Jesus. But for sake of brevity, I will provide only a couple examples. Mark, for example, makes deliberate literary/writing choices in his Gospel account such as the perennial, almost anaphoric use of the word “immediately” (In Greek: εὐθὺς euthys) in his gospel account-—having the effect of depicting Jesus as full of energy and always on the move. Another literary feature Mark employs is an inclusio (a framing device). In the early verses of Mark we read: “And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him: And there came a voice from heaven, saying, Thou art my beloved Son, in whom I am well pleased" (1:10-11, KJV). Note the Greek word for “opened” here is σχιζομένους (root: σχίζω schizō) meaning “to cleave, cleave asunder, rend; to divide by rending; to split into factions, be divided. Also note the words “Thou art my beloved Son.”
Toward the very end of the gospel we read: “And the veil of the temple was rent in twain from the top to the bottom. And when the centurion, which stood over against him, saw that he so cried out, and gave up the ghost, he said, Truly this man was the Son of God. And the veil of the temple was rent in twain from the top to the bottom. And when the centurion, which stood over against him, saw that he so cried out, and gave up the ghost, he said, Truly this man was the Son of God” (15:38-39, KJV). Here is the end of the inclusio as we see the term “rent” ἐσχίσθη which is from the same Greek root word as σχιζομένους / σχίζω schizō we saw at the beginning of the Gospel. And we see the centurion saying “this man was Son of God” completing the inclusio depicting Jesus as God’s son. While you observe the Gospel is “devoid of novel moral material”, I posit that Mark’s theological agenda here is to depict Jesus as the son of God—thus the epitome of morality.
Bauer, Walter, et. al. A Greek-English Lexicon of the New Testament. University of Chicago Press, 1979.
Scroggie, W. Graham. A Guide to the Gospels. Fleming H. Revel Company, 1995.
The Holy Bible, Authorized King James Version, Thomas Nelson, 1972