The Chiddushei HaLev (R' Henoch Leibowitz zt"l) uses an idea we once discussed in a different context (here from the Shem m'Shmuel and here from R' Simcha Zissel) to shed light on a Chazal that we have also discussed a few times (e.g. here), so I thought I would put 2 and 2 together here. Why did Ya'akov make Yosef swear to bury him in Eretz Yisrael? Didn't he trust Yosef? Ramban answers that Ya'akov did not distrust Yosef, but he was aware that Pharoah might not let Yosef carry out his wishes unless he backed them up with the force of an oath. Ramban then adds an additional consideration: Ya'akov knew that the force of an oath would serve as an added motivation for Yosef himself.
Of course Yosef would not try any less to fulfill his father's wishes without that oath. Were Yosef to fail, he would be blameless. However, human psychology being what it is, the fact is that a person works harder when his back is against the wall. A person who says he is giving 100% means he is giving 100% in context, ba'asher hu sham, according to the scale he uses the evaluate his ability at that moment. Put the same person in a dire situation, a do or die, and somehow they discover an extra 10% or 20% to give. When failure is not an option, a person discovers untapped kochos and a person can succeed even where he otherwise would have every justifiable reason not to even make further effort.
The Midrash tells us that had Reuvain known that the Torah would write that he saved Yosef, he would have run home with Yosef on his shoulders. Had Aharon known that the Torah would write that he went out with no jealousy to greet his brother Moshe (our parsha), he would have made a whole parade. Of course Chazal don't mean that the added attention and publicity would have served as a motivator for these giants. What Chazal mean, says R' Henoch, is that had Reuvain known that the Torah itself would testify that he had the koach to save Yosef, that Aharon known that the Torah itself would testify about his ability to overcome natural jealousy, then these giants would have pushed themselves even harder and found the added strength needed to do even what they themselves otherwise had no idea they were capable of.