One casn assume that Peter was described as being given the title of "Magnificent" due to his great leadership, especially in battle. This was a problem in the movie when Aslan simply gave the titles out without the children earning them.
In fact, the only one who was given a reason for having his title was Edmund.
If you really look at their titles combined with what we know about them, one could say that Peter was the High King, the one who made all of the decisions. He also was the main one who led the armies to battle.
Susan was the one who tried to keep the peace, kind of like an ambassador who could go around within Narnia and outside Narnia to try to make allies.
Edmund was the one to whom parties could go in order to settle differences. He was the justice of the land and could act as mediator.
Lucy was the spiritual leader of Narnia. She was the one with the heart of a lion (both figuratively and nearly literally being so close to Aslan) who would never back down from anyone, despite her size.
MrBob