Charles Ellicott Commentary Psalms 103:11

"For as the heavens are high above the earth, So great is his lovingkindness toward them that fear him." — Psalms 103:11 (ASV)

So great is his mercy toward. —Literally, Strong is his mercy upon us (or, over us). (Compare Psalm 117:2.) The comparison in the first clause, and the use of this expression in Genesis 49:26 and 2 Samuel 11:23, suggests as the right rendering here:

For as the heaven is higher than the earth,
So far (above what was expected) for them fearing him prevails his mercy.

(For the same comparison, see Isaiah 55:7-9; Micah 7:19.)