I am Love and Mercy itself. There is no misery that could be a match for My mercy, neither will misery exhaust it, because as it is being granted - it increases. The soul that trusts in My mercy is most fortunate, because I myself take care of it. (Divine Mercy In My Soul, 1273)
I believe His Mercy increases because 1) Jesus is compelled by His Nature to have mercy on the sinner - the sight of his sins compels Him to mercy - so He has even more mercy on the sinner than when he is without sin, 2) Jesus forgives the sinner which is an act of mercy in itself, 3) Jesus gives the soul many graces which is also an act of mercy and because He sees him with His gifts His love for the sinner increases, 4) Jesus is compelled by the sinner’s conversion to give him even more gifts, 5) Jesus is compelled by the sinner’s return to sin to forgive him because He remembers his coming to Him for Mercy and He knows he shall come again to Him for Mercy, and so the sinner’s efforts please Him and He has more mercy on him. And yet even if the soul did not attempt to approach, still His Mercy would increase, because of the poor sinner’s condition; his sins compel Him to have mercy on him. Truly Jesus is God and God is Infinite, and so His Mercy can always increase, without end, and, Jesus is Man and His human mercy - which is divinized due to His glory - can increase just as man’s mercy can increase, although His mercy increases in ways our limited mercy cannot because it is divinized.