Akeidas Yitzchok was the hardest test that Hashem gave to Avraham. Yet Yitzchok knew and also agreed to go along with it. Then why is it considered a bigger test for Avraham than for Yitzchok?
The Tzemach Tzedek, in Derech Mitzvosecha 186b brings this same question in the name of R' Menachem Mendel of Horodok. He asks it much the same as you did, but he adds that Yitzchak was 37, and if he had wanted to.....:
ובזה ית' מה דקשיא טובא מדוע יחס הכתוב נסיון דעקידה לאברהם הלא יותר היה נסיון ליצחק וכדאי' קושיא זו בזהר שהרי יצחק הי' אז בן ל"ז שנה, ואילו לא רצה כו
R' Menachem Mendel answers that the main point of this story is not "mesirus nefesh" (self-sacrifice) at all. The Patriarchs were all "chariots" (completely subservient, as a chariot to the rider) to the Divine Will. Any one of them could, and would (and did), lay down their life for G-d. So the fact that Yitzchak did that is not so amazing. (Plus, as @Menachem says in his answer, Yitzchak may have even asked for this test).
(Jews of all levels have done that throughout our history, including Jews who, up until that moment, hadn't considered themselves Jewish at all. While self-sacrifice is a tremendous thing for us, for a Tzaddik its practically peanuts. Being willing to sacrifice one's son, their only son, whom they love - that's different. Especially for Avraham, who was the embodiment of the attribute of Chesed (kindness), this was in direct conflict with his essence).
R' Menachem Mendel actually adds, that despite that Avraham was willing to sacrifice his son for G-d, the most amazing thing about this story is that Avraham wasn't flustered or confused at all. After all, G-d had previously promised that He would make Yitzchak into a great nation, and now He was asking Avraham to sacrifice him!
וה"ז יכול לחשוב שזהו שינוי רצון וכתיב לא שניתי כו' ואברהם נתחזק ולא הרהר כלל
Avraham might have thought that this was a change in G-d's Will (and G-d has said "I haven't changed"), but nevertheless his faith was strong, and he had no doubts in G-d at all.
This is discussed by Abarbanel (Bereshis 22). He explains previous commentators as understanding that Avraham's pain in killing his own son, although less than Yitzchak's pain of actually being killed, would last throughout the rest of his life, and thus would have been much worse than Yitzchak's pain. In his words:
ואם כן יצחק שמסר עצמו לשחיטה עם היות צער המות שלו יותר גדול מצער אביו שישחטהו הנה צערו לא היה מתמיד כי מיד שישחט לא ירגיש כלום ולא יצטער עוד. אבל הזקן ההורג את בנו יהיה צערו מתמיד ומרת נפשו קודם השחיטה ובעת השחיטה ואחריה כל ימי חייו יום ולילה לא ישבות. ולכן היה ראוי ליחס פלא המעשה הזה לאברחם ולא ליצחק
Yitzchak, who gave himself up for slaughter, even though his pain of death would be greater than that of his father's who would slaughter him, his pain would not be constant and continuing, since after he is killed he will not feel anything; his pain will be over. But the father who kills his son, his pain will continue, and his bitterness before, during, and after the act of slaughter will never rest. Therefore, it is appropriate to attribute the marvel and wonderment of this event to Avraham rather than to Yitzchak.
[I myself, however, disagree with the assumption that Avraham's pain would have been less than Yitzchak's at all. It makes more sense to me, as @zaq wrote in the comments, that it is harder to kill one's "only" son than to be killed himself.]
Abarbanel, however, feels that although the above may be true, Yitzchak should have at least been given some credit while we don't really find that he is given much at all. Therefore, he disagrees with your basic assumption:
Yet Yitzchok knew and also agreed to go along with it. He believes, rather, that Yitzchak was unaware that he was going to be offered as a sacrifice until his father actually tied him onto the mizbeach. Thus, he doesn't really deserve as much credit, since he did not actually go through with the plan willingly.
In a note to the Rada"l's commentary on the Pirkei D'Rebbi Eliezer (Chapter 31), the Rada"l addresses this issue.
He points out the Midrash that Rashi (Bereshit 22:1) quotes:
And some say,“ after the words of Ishmael,” who was boasting to Isaac that he was circumcised at the age of thirteen, and he did not protest. Isaac said to him,“ With one organ you intimidate me? If the Holy One, blessed be He, said to me, ‘Sacrifice yourself before Me,’ I would not hold back.” - Cf. Gen. Rabbah 55:4.
G-d heard this and said, "Since Yitzchok had already agreed to sacrifice himself, this would be a good opportunity to test Avraham through Yitzchok."
This wasn't Yitzchok's test, since he had already verbally requested it (as it says in the Agadah Bereshit).
[I'm not quite sure which sefer the Agadah Bereshit is]
The Malbim (synopsis here in Sefer HaCarmel) explains that the primary challenge of the akeida was actually not the command to sacrifice Yitzchok, but rather the command to not sacrifice him (see the link for what led him to this conclusion). This is because the challenge for Avrohom was not the actual act of sacrificing Yitzchok, but was rather to have the lofty intentions of doing it purely for G-d while doing it. As for sacrificing Yitzchok, what other intentions would Avrohom have had other than for the sake of the command he was given? However, when it came to the command to not sacrifice him, Avrohom had a tremendous challenge that even this, which was completely in line with his own desires, should be done with pure intentions to only do it because Hashem had commanded so.
In a note, the editor (R' Yosef Greenbaum) writes that this answers the question of the Machazis Hashekel (who asks your question) - namely, that Yitzchok may have been greater in his willingness to be sacrificed, but in the more significant part of the test, having in mind only the desire of Hashem even when fulfilling his own wish, Avrohom was the only participant (since Yitzchok was bound up at that point and had no choice in being killed or not).
As an aside, the Machazis Hashekel's own answer, which he quotes from the sefer Nezer HaKodesh, is the same as the Abarbanel quoted by @Jake, namely that Yitzchok's suffering would be momentary while Avrohom's anguish would be for the rest of his life.
R' Yaakov Kaminetzky has a long discussion about this. Part of what he says is that the akeida went against Avraham's nature, which was chessed/kindness. Yitzchak's attribute was gevurah and is closely related to self-sacrifice, so this test was really "right up his alley." Therefore, it was a harder test for Avraham, as it opposed his nature.
I would argue with the assumption that it was a bigger test for Avraham (unless you have a source other than those commentaries that accept the assumption in answering the question). Twice in the tefillos of the Yamim Noraim we refer to the akeida as a zechus of Yitzchok ("u'vben hane'ekad yashbis m'dayneinu" and "v'akeidas Yitzchok lzaroh hayom brachamim tizkor") and don't connect it to Avraham, and no other zechus is related to Yitzchok - it seems to be his biggest. (Acc. to the Ramban, Ibn Ezra who assumes Yitzchak was too young to fight back for the reasons that led to the question either changed or had a different version of those lines of the tefillah.)
R. Bachye Ben Asher in his commentary to Genesis 22:7 explains that it is easier to give your own life up than to kill your son, and he cites an interesting proof for this:
יש שאמרו גדול כחו של אברהם יותר מיצחק לפי שהמפקיר עצמו ליהרג נקל הוא מאד מן האב שבא לשחוט את בנו בידיו אם לא תאמר כן למה צוה הקב"ה לאברהם לשחוט את בנו בידיו יצוהו להרוג את עצמו או שתבא המצוה ליצחק אלא בודאי זה ראיה כי גדול כחו של אברהם יותר מיצחק
Some say that Abraham's strength was greater than Isaac's, since giving oneself up to be killed is much easier than a father that comes to slaughter his son with his [own] hands. If you don't say this, why did God command Abraham to slaughter his son with his hands? He should have commanded him to kill himself, or the command should have come to Isaac. Rather this is certainly a proof that the strength of Abraham was greater than Isaac's.
In other words, if giving yourself up to be killed was harder than killing your son then that would have been the test! From the fact that the test was for Avraham to sacrifice his son, we can infer that that was harder.