Why does it say "Birkas Hashem Hi Sa'ashir"?
R. Yonah: It is what causes the rich to be rich.
Malbim: Wealth that comes via efforts is not true wealth. Its owner eats "Lechem ha'Atzavim" (Tehilim 127:2) - when he eats bread he is sad, for he recalls his toil to get it, and worries lest he lack. Therefore he skimps on his food, as if this will increase his wealth 1 . This is because it is not true wealth. He considers himself poor. Birkas Hashem gives true wealth. He need not sadden himself to add to his wealth. He eats b'Simchah and in abundance, for Brachah is found in all that he has.
Why does Malbim say that this will not increase his wealth? Hashem decreed what he will earn in the year (Beitzah 16a). If he overspends, he will lack (Rashi 16a). If he skimps, more will remain! (PF)
Why does he not add Etzev?
Rashi: He need not exert to become rich. The Brachah that Hashem gave to him suffices for him.
R. Yonah: Hashem's Brachah will not be accompanied with pain and worry about the money - "Yiten li'Ydido Shena" (Tehilim 127:2). This refers to the Tzadik mentioned. A Tzadik is not frantic to [amass] wealth. He is not pained over lack of his wealth. He knows that if Hashem will command Brachah, he will not need pain and distraction [to get it]. His Brachah will come without many thoughts. What Hashem does not decree to give to him, many thoughts will not help - "Shav Lachem Mashkimei Kum Me'acharei Sheves Ochelei Lechem ha'Atzavim" (ibid.) A Tzadik is happy with what Hashem gave to him. He knows that pain will not add money to what Hashem blessed him. Etzev is pain and anger - "u'Devar Etzev Ya'aleh Af" (below, 15:1).
Malbim: Refer to 10:22:1:2.
