To abound is to have a surplus, and abundance, and extravagant portion.
Should the fish in the Pacific worry that it will run out of ocean? No. Why? The ocean abounds with water. Does the Seagull need to be anxious about finding room in the sky to fly? No. The sky abounds with space. Should the Christian worry that the cup of mercy will run empty? He may. For he may not be aware of gods abounding grace. Are you? Are you aware that the cup God gives you overflows with Mercy? Or are you afraid your cup will run dry? Your warranty will expire? Are you afraid your mistakes are too great for God's grace?… God is not a miser with his grace. Your cup maybe low on cash or clout, but it is overflowing with Mercy.
And you can take that from Romans 5:20. The more we see our sinfulness, the more we see God's abounding grace.