Shiluach HaKen as a light mitzvah

In parshat Ki Teitzei, the following pasuk:

ו  כִּי יִקָּרֵא קַן-צִפּוֹר לְפָנֶיךָ בַּדֶּרֶךְ בְּכָל-עֵץ אוֹ עַל-הָאָרֶץ, אֶפְרֹחִים אוֹ בֵיצִים, וְהָאֵם רֹבֶצֶת עַל-הָאֶפְרֹחִים, אוֹ עַל-הַבֵּיצִים--לֹא-תִקַּח הָאֵם, עַל-הַבָּנִים.

6 If a bird's nest chance to be before thee in the way, in any tree or on the ground, with young ones or eggs, and the dam sitting upon the young, or upon the eggs, thou shalt not take the dam with the young;

Rav Yitzchak Karo, the uncle of the Mechaber, wrote:

כִּי יִקָּרֵא קַן-צִפּוֹר לְפָנֶיךָ -- there is to ask, a light mitzvah such as this, which has no monetary loss at all, nor bodily effort, why is its reward so great, that לְמַעַן יִיטַב לָךְ, וְהַאֲרַכְתָּ יָמִים. And further, to a world that is completely tov and a world that is completely aruch.

And the answer in this is that the reward for the lightest of the mitzvot is very great, for the {hidden} treasure of the mitzvot, who sees them and who knows their greatness? For it is without limit, no eye has seen it except for that of God. And all the more so by virtue of the commander, the king, king of kings, Hakadosh Baruch Hu. And from here we will recognize the elevated nature of the more severe commandments.

And some say that the sar {heavenly prince} appointed upon the birds cries out before Hashem, because of its mercy upon the mother who is calling out and crying upon her children, and Hashem dresses himself in mercy upon his sons who were exiled between the nations. For he says "If a bird weeps and cries upon its children, kal vachomer I upon my sons." And it is written {Yirmeyahu 13:17}:

יז  וְאִם לֹא תִשְׁמָעוּהָ, בְּמִסְתָּרִים תִּבְכֶּה-נַפְשִׁי מִפְּנֵי גֵוָה; וְדָמֹעַ תִּדְמַע וְתֵרַד עֵינִי, דִּמְעָה, כִּי נִשְׁבָּה, עֵדֶר יְהוָה.  {ס}

17 But if ye will not hear it, my soul shall weep in secret for your pride; and mine eyes shall weep sore, and run down with tears, because the LORD'S flock is carried away captive. {S}

because of the pride of Israel which was humbled. And is appears that for that reason it did not state by kibud av vaEm the words יִיטַב לָךְ,  but rather yaarichun yamecha, while by shiluach kan tzipor is wrtten  יִיטַב לָךְ, because Hashem will be filled with mercy for his children."

In terms of the light mitzvah with the same reward as the heavy mitzvah, I've always understood this as what seems, from our limited human perspective, to be light and heavy, for the true value of each mitzvah is beyond our ken. Just as one might correctly classify the severity of prohibitions on the basis of their penalty. Alternatively, this is indeed a light and a heavy mitzvah, respectively, but Hashem declared the reward for each to be identical so that in general, we don't start assessing mitzvos in this manner and come to treat the light ones lightly.

In terms of the kabbalistic reason of causing Hashem to have mercy on us, see Rabbi Natan Slifkin's essay on Shiluach haKein, the Transformation of a Mitzvah.