Jacob explained to him that there is a difference between Ephraim and Menashe, that Ephraim's greatness will be a greatness of quality, and Menace's greatness will be a greatness of quantity, meaning that sometimes an individual is the equivalent of an entire nation due to their level and strength of their holiness...like we see in the Midrash that Rabbi Akiva interpreted that one woman bore in Egypt 60,000 children, about Moses our Teacher who was equivalent to all of Israel - this is greatness of quality, and this will be seen in Ephraim, as Rashi taught, who seed like be like a nation - this is Joshua, but the greatness of Menasheh will be through great quantity, and it is explained that the greatness of Ephraim is a supernatural matter, and depends on miracles and wonders that were done by Joshua, and therefore Jacob blessed him with his right hand, but the greatness of Menasheh depends on a large people, which is a natural blessing, related to the left.