Ki Teitze תרל”ב 2

במדרש שמצינו קלה שבקלות חמורה שבחמורות מתן שכרה שוה.
The Midrash says that HaShem didn’t want to reveal the reward of the mitzvos, and therefore the only two that are revealed. One is a very easy mitzvah – shiluach haken sending away the mother bird, and the other is a very difficult mitza – honoring one’s parents. And both rewards in the Torah are the same, blessed with a long life.
וקשה למה באמת כן.
And it’s difficult why is it indeed structured this way, why should a difficult and easy mitzvah – have the same reward?
וי”ל פשוט כי מה שהמצוה גדולה נותנת כח וחשק לאדם לקיימה
We can answer simply that a person has more desire to do a difficult mitzvah
כמ”ש במד’ שאם הי’ נגלה שכרן היו רצין לחמורות בלבד.
As the Midrash says that if we would know the reward of the mitzvos people would only do the difficult ones and not the easy ones (because they want more reward). So we see that even though it is more difficult nevertheless the desire to do it is stronger.
ולכך הקלה קשה יותר לקיימה ומה”ט מתן שכרן שוה כנ”ל:
And therefore the easy mitzvah- is actually more difficult to do than the hard one. Therefore their reward is the same!

(This can be further explained by what the Ba’al Shem Tov says on the Mishnah in Avot (4:2) : run to a difficult mitzvah as you would to a light mitzvah, because you do not know the reward of any mitzvah. This Mishna is seemingly contradictory. If you know which one is difficult and which one is light then you do know the reward?
He answers that there are 2 elements in the mitzvah, the one is the effect of each individual mitzvah and these are indeed different, the other element is doing a mitzvah because this is HaShem’s will, in this part all mitzvahs are equal. This is part of the mitzvah that the Sfas Emes is referring to.)