I found this description over at monachos.http://www.monachos.net/conversation/topic/6682-little-man-on-the-back-of-st-georges-horse/
The "little man" probably symbolizes a posthumous miracle reputedly performed by St. George:
During their invasion of Paphlagonia the Agarenes (Moslems) took many people into captivity, among them a young boy who was a servant in the church of St. George in Phatris. Some of the prisoners were killed, the rest turned into slaves. The boy was of such beauty that he was chosen as a servant for the Arabian ruler. As he rejected the offer to become a Muslim, he was sent to work in the kitchen. In his misfortune the poor boy prayed to Saint George. Once at evening, when he was lying in bed, he heard a voice coming from the yard and calling his name. The boy opened the door and saw a rider who caught him and placed behind himself on the horse. Then the steed rushed forward and started to gallop. The rider brought the boy to a certain building, and then disappeared. The exhausted youth fell asleep and next morning was awakened by the people, who were dismayed because his Arabian clothes suggested the presence of enemies. The boy recognised those people as monks. As it transpired, he had been brought to Monastery of St. George. All of them went to a church to offer a thanksgiving prayer to God for saving the youth.
Another version of the story has the young man miraculously returned to his parents after they pray to St. George.