The tone of "The Knight's Tale" is epic, meaning high and grand, because many things are described in it as the best/greatest/most they could possibly be. Theseus, for example, is such a great conqueror that "gretter was ther noon under the sonne." When Arcite returns to Athens, the narrator tells us that "so much sorwe hadde nevere creature / That is, or shal whil that the world may dure" (501-502). Never have shields been shinier, spears sharper, nor men more battle-worthy than those knights that fight alongside Palamon and Arcite.
The point is that everything that happens in "The Knight's Tale" feels larger than life. This epic tone is the narrator's way of convincing us of the importance and worthiness of his subject matter.