St. John the Baptist, in John 3:30, demonstrates his profound humility as the Precursor to Christ. Another example demonstrating St. John the Baptist's profound humility is Mt. 3:11:
…he that shall come after me, is stronger then I, whose shoes I am not worthy to bear…
The Catholic Haydock Commentary
has this to say about John 3:30:
Ver. 30. He (Christ) must increase, not in virtue and perfection, with which he is replenished, but in the opinion of the world, when they begin to know him, and believe in him: and in like manner, I must be diminished, when they know how much he is above me. (Witham)
St. Thomas Aquinas's Catena Aurea (Golden Chain), a collection of excerpts from the Fathers' commentaries on the Gospels, says:
St. CHRYSOSTOM. He next dismisses the motions of envy, not only as regards the present, but also the future, saying, He must increase, but I must decrease: as if he said, My office has ceased, and is ended; but His advances.
St. AUGUSTINE. What means this, He must increase? God neither increases, nor decreases. And John and Jesus, according to the flesh, were of the same age: for the six months’ difference between them is of no consequence. This is a great mystery. Before our Lord came, men gloried in themselves; He came in no man’s nature, that the glory of man might be diminished, and the glory of God exalted. For He came to remit sins upon man’s confession: a man’s confession, a man’s humility, is God’s pity, God’s exaltation. This truth Christ and John proved, even by their modes of suffering: John was beheaded, Christ was lifted up on the cross. Then Christ was born, when the days begin to lengthen; John, when they begin to shorten. Let God’s glory then increase in us, and our own decrease, that ours also may increase in God. But it is because you understand God more and more, that He seems to increase in you: for in His own nature He increases not, but is ever perfect: even as to a man cured of blindness, who begins to see a little, and daily sees more, the light seems to increase, whereas it is in reality always at the fall, whether he sees it or not. In like manner the inner man makes advancement in God, and it seems as if God were increasing in Him; but it is He Himself that decreases, falling from the height of His own glory, and rising in the glory of God.
St. THEOPHYLACT. Or thus; As, on the sun rising, the light of the other heavenly bodies seems to be extinguished, though in reality it is only obscured by the greater light: thus the forerunner is said to decrease; as if he were a star hidden by the sun. Christ increases in proportion as he gradually discloses Himself by miracles; not in the sense of increase, or advancement in virtue, (the opinion of Nestorius,) but only as regards the manifestation of His divinity.