Our Lord says that the Father is greater than Him because (1) the unbegotten Father eternally generates the Son, not the other way around; and (2) the Son assumed human nature and so subsists not only "in the form of God" [Phil 2:6] but also in "the form of a servant" [Phil 2:7].
Bishop St. Gregory the Wonderworker of Neocaesarea, Homily 4 On the Holy Theophany: "If He shall say, 'I and my Father are one' [John 10:30], hear Him. If He shall say, 'He that has seen Me has seen the Father' [John 14:9], hear Him. If He shall say, 'He that has sent Me is greater than I' [John 14:28], adapt the voice to the economy."
Archbishop St. Alexander I of Alexandria, Epistle to Archbishop St. Alexander I of Constantinople qtd. in Bl. Bishop Theodoret of Cyrus, Ecclesiastical History 1:3:
Therefore His own individual dignity must be reserved to the Father as the Unbegotten One, no one being called the cause of His existence: to the Son likewise must be given the honor which befits Him, there being to Him a generation from the Father which has no beginning; we must render Him worship, as we have already said, only piously and religiously ascribing to Him the "was" and the "ever," and the "before all ages;" not however rejecting His divinity, but ascribing to Him a perfect likeness in all things to His Father, while at the same time we ascribe to the Father alone His own proper glory of "the unbegotten," even as the Savior Himself says, "My Father is greater than I" [John 14:28].Archbishop St. Athanasius I the Great of Alexandria (Doctor), :
Bishop St. Hilary of Poitiers (Doctor), On the Trinity 7:6:
Bishop St. Ambrose the Great of Milan (Doctor), Exposition of the Christian Faith 2:8:59:
It was due to His humanity, therefore, that our Lord doubted and was sore distressed, and rose from the dead, for that which fell does also rise again. Again, it was by reason of His humanity that He said those words, which our adversaries use to maliciously turn against Him: "Because the Father is greater than I" [John 14:28].Bishop St. Basil the Great of Caesarea (Doctor), Letter 8:5 to the Caesareans:
And again, "My Father is greater than I" [John 14:28]. This passage is also employed by the ungrateful creatures, the brood of the evil one. ... even from this passage the consubstantiality of the Son with the Father is set forth. For ... comparisons may properly be made between things which are of the same nature. We speak of angel as greater than angel, of man as juster than man, of bird as fleeter than bird. If then comparisons are made between things of the same species, and the Father by comparison is said to be greater than the Son, then the Son is of the same substance as the Father. But there is another sense underlying the expression. In what is it extraordinary that He Who "is the Word and was made flesh" [John 1:14] confesses His Father to be greater than Himself, when He was seen in glory inferior to the angels, and in form to men? For "You have made Him a little lower than the angels" [Psalm 8:6], and again "Who was made a little lower than the angels" [Hebrews 2:9], and "we saw Him and He had neither form nor comeliness, His form was deficient beyond all men" [Isaiah 53:2]. All this He endured on account of His abundant loving kindness towards His work, that He might save the lost sheep and bring it home when He had saved it, and bring back safe and sound to his own land the man who went down from Jerusalem to Jericho and so fell among thieves. ... This is why the Son is less than the Father; for your sakes He was made dead to free you from death and make you sharer in Heavenly life.Bishop St. Augustine the Great of Hippo (Doctor of Grace), On the Predestination of the Saints 2:67:
The same great bishop says in On the Trinity 1: ,
Moreover, in Tractate 78:1 on the Gospel of St. John, St. Augustine says
Pope St. Leo I the Great of Rome (Doctor), Sermon 23:2 On the Feast of the Nativity:
This union, dearly beloved, whereby the Creator is joined to the creature, Arian blindness could not see with the eyes of intelligence, but, not believing that the Only-begotten of God was of the same glory and substance with the Father, spoke of the Son's Godhead as inferior, drawing its arguments from those words which are to be referred to the "form of a slave" [Phil 2:7], in respect of which, in order to show that it belongs to no other or different person in Himself, the same Son of God with the same form, says, "The Father is greater than I" [John 14:28] just as He says with the same form, "I and my Father are one" [John 10:30]. For in "the form of a slave" [Phil 2:7], which He took at the end of the ages for our restoration, He is inferior to the Father: but in the form of God, in which He was before the ages, He is equal to the Father.The same great pope says in his Tome (Letter 28:4 to Archbishop St. Flavian the Martyr of Constantinople),
Just as therefore, to pass over many other instances, it is not part of the same nature to be moved to tears of pity for a dead friend, and when the stone that closed the four-days' grave was removed, to raise that same friend to life with a voice of command: or, to hang on the cross, and turning day to night, to make all the elements tremble: or, to be pierced with nails, and yet open the gates of paradise to the robber's faith: so it is not part of the same nature to say, "I and the Father are one" [John 10:30] and to say, "the Father is greater than I" [John 14:28]. For although in the Lord Jesus Christ God and man is one person, yet the source of the degradation, which is shared by both, is one, and the source of the glory, which is shared by both, is another. For His manhood, which is less than the Father, comes from our side: His Godhead, which is equal to the Father, comes from the Father.Hieromonk St. John of Damascus (Doctor), An Exact Exposition of the Orthodox Faith 4:18:
And others make known the fact of His origin from the Father as cause, for instance "My Father is greater than I" [John 14:28]. For from Him He derives both His being and all that He has: His being was by generative and not by creative means, as, "I came forth from the Father and have come" [John 16:28], and "I live by the Father" [John 6:57]. But all that He has is not His by free gift or by teaching, but in a causal sense, as, "The Son can do nothing of Himself but what He sees the Father do" [John 5:19]. For if the Father is not, neither is the Son. For the Son is of the Father and in the Father and with the Father, and not after the Father. In like manner also what He does is of Him and with Him. For there is one and the same, not similar but the same, will and energy and power in the Father, Son and Holy Spirit.