Why does the Torah mention that Efrayim was the younger or smaller one?
What does the Torah mean when it writes that Ya'akov guided his hands "Ki (because) Menasheh was the B'chor"?
Rashi (citing Targum Onkelos) and Rashbam: The Torah is pointing out that Ya'akov switched his hands deliberately, precisely because he knew that Menasheh was the B'chor (and that Yosef had placed him on his right).
Rosh: He switched his hands rather than the boys, leaving Menasheh on the right because he was the B'chor.
Hadar Zekenim (18): Some say 'even though Menasheh was the Bechor', just like "la'Shem
Riva rejects this. 'Even though' is not one of the four meanings of 'Ki'!