Why does the Pasuk mention "in that land," and "in that year," seeing as it is obvious?
Rashi: To inform us that, even though it was not Eretz Yisrael proper 1 (i.e. the land of the seven nations), and even though it was a year of famine, Yitzchak enjoyed a bumper harvest (100 times more than anticipated).
Seforno: To tell us that HaSh-m's words and blessing - that he should reside in that land and that He would bless him, came true. 2
Moshav Zekenim: It was in Chutz la'Aretz. He went only to seed there, but not to live there.
Since the land was in a state of famine, why did Yitzchak bother to sow his fields?
Tosefta (Berachos, Perek 6): Based on the verse "u'Verachticha v'Hirbeisi es Zar'acha" (26:24), 1 Yitzchak understood that Berachah will only come about through the work of his hands, so he proceeded to plant crops. 2
What does it mean that [the land] produced "Me'ah She'arim"?
Rashi and Rashbam: They estimated how much it will produce, for the sake of Ma'aser. It produced 100 times as much. 1
Rashi (to Kesuvos 112a): It produced 100 times more than it did in a regular year. 2
Seforno: It was because he gave Ma'aseros that HaSh-m blessed him, as the Pasuk says, "Bring all the Ma'asros to the storehouse... and see if I don't open up the windows of the Heaven for you and shower you with blessing without end!" (Malachi 3:10). Oznayim la'Torah - This is as the Gemara states (Ta'anis 9a), 'Aser, Bishvil she'Tis'asher' (Give Ma'aser in order to become wealthy). This also explains why the Torah states, "va'Yevarechehu HaSh-m" after "va'Yimtza ba'Shanah ha'Hi Me'ah She'arim;" and not before it.
Why does the Torah need to add that "HaSh-m blessed him," seeing as this is self-evident?
Seforno: Not only did he enjoy a bumper harvest, but he was able to sell his crops at a large profit.
Malbim #1: He ate little, and the food was blessed in his innards, and he was satiated.
Malbim #2: He succeeded in all his endeavors, like the verse, "va'Yehi Ish Matzli'ach" (39:2).
Ha'amek Davar: It was a famine year, and people of the area needed to buy grain from him, unlike the king's desire (refer to 26:11:152:2).
Ohr ha'Chayim: There was no Ayin ha'Ra against him, even though he prospered so much.
QUESTIONS ON RASHI
Rashi writes: It produced 100 times as much as the estimation for Ma'aser. What does this mean?
Chizkuni: Ma'aser (a 10th) of the harvest was 100 times the estimate.
Riva: A Beraisa (Tosefta Berachos 6:8) implies that the entire harvest was 100 times the estimate! Bereishis Rabah (64:6) - he measured the harvest in order to tithe it. 1
Hadar Zekenim: Why would he estimate [the harvest]? Berachah does not rest on what is estimated! 2 It was a famine year, and he estimated how much Ma'aser 3 there will be, in to give to Aniyim promptly.
One should not tithe by estimation (Avos 1:16).
Our text (Ta'anis 8b, Bava Metzia 42a) says 'Madud' (measured). It seems that Hadar Zekenim's text said 'Amud' (estimated). Surely he agrees that also (or all the more so) Berachah does not rest on what is measured; the Gemara says so about what is counted! He must hold that the Torah tells us the amount of the harvest, but Yitzchak himself did not measure it.
Rashi writes: "'One hundred times' - ... it produced 100 times as much as had been projected. But our Sages said, this calculation was for [separating] tithes." It seems there is no contradiction between the two explanations; why does Rashi cite the Midrash of the Sages?
Gur Aryeh: In its simple sense, why did Yitzchak calculate the yield at all? Blessing comes only to that which is hidden, not that which is measured or counted (Ta'anis 8b). Rather, Yitzchak calculated the yield in order to separate Ma'aser.