He declares this with certainty, that there is no one greater than John. But by saying “born of women” He excludes Himself, for Christ was born of a virgin, not of a woman, that is, one who is married. Notwithstanding, He that is younger is greater than he in the kingdom of heaven. Since He has extolled the praises of John, lest they think that John is greater than He says here more clearly, I am the younger in age and the lesser in your opinion, yet I am greater than he in the kingdom of heaven, that is, in regards to spiritual and heavenly good things. For here I appear less than he, both because his birth preceded Mine and because he appears great to you, but there in the kingdom of heaven I am greater.
Plus the Catena Aurea has great quotes
RABAN; As much as to say; What need to recount one by one the praises of John the Baptist; I say verily to you, Among them that are born of women &c. He says women, not virgins. If the same word mulier, which denotes a married person, is any where in the Gospels applied to Mary, it should be known that the translator has there used ‘mulier’ for ‘femina’; as in that, Woman, behold your son!
JEROME; He is then set before all those that are born in wedlock, and not before Him who was born of the Virgin and the Holy Spirit; yet these words, there has not arisen a greater than John the Baptist, do not imply that John is to be set above the Prophets and Patriarchs and all others, but only makes him equal to the rest; for it does not follow that because others are not greater than him, that therefore he is greater than others.
PSEUDO-CHRYS; But seeing that righteousness has so great deepness that none can be perfect therein but God only, I suppose that all the saints tried by the keenness of the divine judgment, rank in a fixed order, some lower, some before other. Whence we understand that He that has none greater than Himself, is greater than all.
CHRYS; That the abundance of this praise might not beget a wrong inclination in the Jews to set John above Christ, he corrects this, saying, He that is least in the kingdom of heaven is greater than he.
AUG; The heretic argues from this verse to prove, that since John did not belong to the kingdom of heaven, therefore much less did the other Prophets of that people, than whom John is greater. But these words of the Lord may be understood in two ways. Either the kingdom of heaven is something which we have not yet received, that, namely, of which He speaks, Come, you blessed of my Father, receive the kingdom, because they in it are Angels, therefore the least among them is greater than a righteous man who has a corruptible body. Or if we must understand the kingdom of heaven of the Church, whose children are all the righteous men from the beginning of the world until now, then the Lord speaks this of Himself, who was after John in the time of His birth, but greater in respect of His divine nature and supreme power. According then to the first interpretation it will be pointed, He who is least in the kingdom of heaven, is greater than he; according to the second, He who is less than he, is in the kingdom of heaven greater than he.
CHRYS; The kingdom of heaven, that is, in the spiritual; world and all relating thereto. But some say that Christ spoke this of the Apostles.
JEROME; We understand it simply, that every saint who is ready with the Lord is greater than he who yet stands in the battle; for it is one thing to have gained the crown of victory, another to be yet fighting in the field.