Why does it say "Gadol Ad Shamayim Chasdecha"?
Radak: I thank You, for the Chesed that You did with me is great Ad (until) Shamayim 1 - "Ki ch'Govah Shamayim Al ha'Aretz Gavar Chasdo Al Yere'av" (103:11). I already explained Chesed and Emes (refer to 57:4:4:1).
Malbim (here and 36:6): Nature is drawn after man. Men receive Brachos or curses, based on their deeds, so Chesed (miraculous conduct) is also in Shamayim, not only in Shechakim. "Rumah Al ha'Shamayim Elokim" (verse 6) corresponds to "Ekra lEi'lokim Elyon" (verse 3) - He is the First of all causes, which come from Shamayim and the Ma'arachah. Everything is from Hashgachah, also in Shamayim!
Why does it say "v'Ad Shechakim Amitecha"?