The Anointment of David as King
Metropolitan Museum of Art
This appears to be a conflation of the Bible's three narratives of the anointing of David. In the first, 1 Samuel 16:1-13, David is a shepherd boy anointed by the prophet Samuel. In the present image, David's size could indicate his youth, and the haloed figure blessing the boy with a Trinitarian gesture ought to be Samuel.
But everyone except Samuel is dressed for battle, and Samuel is not doing the anointing, suggesting that the event depicted is either David's royal anointing by either the men of Judah (2 Samuel 2:4) or the men of Israel (2 Samuel 5:1-5).
But the latter passage says that David was 30 years old at the time; and Samuel was long dead.
One might preserve appearances by concluding that the small size of the David figure represents his relative youth at the time of his anointing by the men of Israel (he would reign for another 40 years), and that Samuel is included because his anointing of David was the real one, the one that mattered because it had been directly ordered by God.
More of David
Photographed at the Metropolitan by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.