1)
Why does it say "Rachamecha Rabim"?
1.
Radak: Because many pursued him, like the coming verse says, it says that Your mercy is great. I.e. it will save me from them.
2.
Malbim: In addition to asking Him to sustain him like His word, David said that since Hashem's mercy is great, it is proper also that he sustain him according to His Mishpat and Din, based on his merits.
2)
What is "k'Mishpatecha"?
1.
Radak: This is like Your good conduct (refer to 119:132:1:1).
