God's mercy is great. His chesed for all of us, His Covenant kindness for us is so wonderful.
Yet did you know that it is actually expanding?
His Covenant kindness to us is not a fixed entity, but never-ending, even growing!
"Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound:" (Romans 5:20)
Scientists have no affirmed that the heavens, space all around the earth and throughout the universe, is actually expanding, too!
So, the Pslamist is not taking about a fixed albeit grand mercy that God shows to us.
If that is not enough, look at the verse which follows:
"As far as the east is from the west, so far hath he removed our transgressions from us." (Psalm 103:12)
How far is east from west? In order to establish one direction, we have to go further in the other direction. The more that we try to establish that other direction the further we must continue to go.
In other words, the two directions are forever distant, and to ensure that accuracy of that distance, we have to keep going and going in one of those directions!
God has not only removed our sins, but they keep being removed further and further away from us. Their distance from us can never be pinned down.
God's promise to us regarding our sins is as ironclad as His promise affirmed by the rainbow:
"9For this is as the waters of Noah unto me: for as I have sworn that the waters of Noah should no more go over the earth; so have I sworn that I would not be wroth with thee, nor rebuke thee.
"10For the mountains shall depart, and the hills be removed; but my kindness shall not depart from thee, neither shall the covenant of my peace be removed, saith the LORD that hath mercy on thee." (Isaiah 54: 9-10)
So, remember that God's grace is indeed so great that we cannnot take it in, and yet its greatness cannot be measured, cannot be contained.
Like the heavens which are boundless and growing, and like the distance between East and West which is never-ending, so to is God's grace toward us .. superabounding!