Question: How could Jesus be God when he said, "the Father is greater than I" (John 10:29)?
Answer: He called the Father greater because Jesus was speaking of his role of submitting to Him. "Greater" is a quantitative term of position, not a qualitative term of nature. Even though people are equal in nature, some are greater than others.
The President of the United States is greater than everyone in the country due to his high position. Jesus said, "You know that those who are recognized as rulers of the Gentiles lord it over them; and their great men exercise authority over them. But it is not so among you, but whoever wishes to become great among you shall be your servant." (Mark 10:42-43). Although these people are equal in nature, some are greater than others.
In Hebrews 1:4, Jesus is described as "betterthan the angels." "Better" is a qualitative term of nature, not a quantitative term of position. When Jesus humbled Himself by leaving heaven and coming to earth (Phil. 2:6-8), the Father became greater than Him. "For a little while" (during His time on earth) He became "lower than the angels" (Heb. 2:7). Even though He took this lower position for a little while, He kept His nature of deity because Heb. 1:6 says, "Let the angels of God worship Him."
So the Father was "greater" than Jesus while on earth, but not "better" because Jesus was God in the flesh.