midrash - Why was Akeidas Yitzchok a bigger test (Nisayon) for Avraham than for Yitzchok?

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?

Answer

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.