What is the meaning of "Mah Enosh"?
Radak: This is opposite to "Mah Adir Shimcha" (verse 2), which aggrandizes; "Mah Enosh" belittles [man].
Malbim: "Enosh" implies smallness - the size of his body, his weakness due to his lowly place, his natural physical weakness and weak composition.
Why did he say "Ki Sizkerenu
Radak: Why did You put a 'copy' of the Hod of the spiritual into man? Man is so inferior to the celestial bodies!
Why did he say "u'Ven Adam Ki Sifkedenu"?
Radak #1: The matter is repeated in different words.
Radak #2: "Sizkerenu" is putting a 'copy' of Your Hod into man. Sifkedenu is Hashgachah - giving to man according to his deeds.
Malbim: 'Ben Adam' demonstrates that he does not last forever. He is born from a human; the species endures, but not the person.