What do we learn from "Mesukah Shenas ha'Oved Im Me'at v'Im Harbe Yochal"?
Menachos 110a: It does not matter whether one does (offers) much or little, as long as he intends l'Shem Shamayim 1 .
Rav Sadya Gaon: The worker's sleep 2 is sweet, because he eats what he has, be it much or little (his mind is not distracted).
Rashi citing Tanchuma Ki Sisa 3, #1: One who works the land, his sleep is sweet, no matter how much he eats, for he is used to this.
Rashi citing Tanchuma Ki Sisa 3, #2: One who serves Hashem, whether his years are many or few, he will eat the reward of the little like the much. Moshe led Yisrael for 40 years, and Shmuel ha'Navi for 10, and a verse equates them - "Moshe v'Aharon b'Chohanav u'Shmuel b'Kore'ei Shmo" (Tehilim 99:6).
Rashbam: The year and time of the farmer is good. He can eat from working his land, whether little or much.
Ri Kara: This one who did not gathered storehouses of silver and gold, and he works his land enough to feed his household, he is better than one who gathered much silver and gold. When the worker returns at night from the field, his sleep is sweet.
Rid: Even though there is no gain from working the land (refer to 5:10:1:6), it is better than pursuing wealth.
Seforno: The servant enjoys his sleep more than the Ashir enjoys his sleep.
Metzudas David: Even if he eats little, he sleeps as if he ate much, which normally brings sleep. Because he has only work in the field, he has nothing to think about to lose sleep over it.
Ibn Ezra: There is a Patach under the Nun in Shenas, because it is Samuch. "V'Im Eten Shenas l'Einai" (Tehilim 132:4) has Kamatz (according to Ibn Ezra's text); it is not Samuch, due to the prefix Lamed in l'Einai. Ramach says that it is like Shenasi, similar to "Avi v'Zimras Kah" (Shemos 15:2).
Why does the Ashir not sleep?
Rav Sadya Gaon: He does not enjoy his sleep.
Rashi #1: One who does much business, the entire night he thinks about it.
Rashi #2: This is one who knows many teachings. He does not sleep in the grave. "Dovev Sifsei Yeshenim" - any Chacham, that people recite his teachings, his lips move in the grave.
Rashbam: He always thinks about his money, lest he lose it 1 - "Gam ba'Laylah Lo Shachav Libo" (2:23). Ibn Ezra - he fears day and night from the king, extortionists and thieves. "Sava" is his great wealth 2 . At night he thinks how to satiate his Nefesh from his wealth. Alternatively, it refers to food, like "Pen Tisba'enu va'Hakeiso" (Mishlei 25:16).
Seforno: He is worried lest he lose his money or part of it, and he thinks about financing his large household.