The detractors are referring to a statement Joseph Smith delivered in Nauvoo, Illinois, in May, 1844, shortly before his death. It is true that he was boasting, having patterned his address after a talk by Paul recorded in 2 Corinthians chapter 11. In that sermon, Paul the Apostle was doing some boasting of his own to the Gentiles. Joseph Smith picked up on Paul's theme when he said,
I have more to boast of than any man had. I am the only man that has ever been able to keep a whole church together since the days of Adam. A large majority of the whole have stood by me. Neither Paul, John, Peter, nor Jesus ever did it. I boast that no man ever did such work as I (History of the Church, Vol.6, pp. 408-09).While we aren't sure this is a completely accurate quote, let us assume that it is. Detractors read into this statement that the Prophet was saying he did a greater work than Jesus Christ. Considering the entire text and the circumstance of the time, he seems to be saying only that he was able to keep the Church together better than others did, including Jesus Christ. Surely Joseph Smith would be the first to agree that keeping a church together is not a greater or a more significant work than what was done by Jesus.
There is nothing as significant as being the God of Israel, taking upon oneself the sins of the world, dying for all mankind that they might live, nor being resurrected. Surely, everlasting life is the greatest gift anyone could give.
Nevertheless, the Lord himself said, "Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father" (John 14:12). To what greater work could the Savior be referring? Perhaps the Lord means a larger work, but certainly not more significant. For example, John the Baptist presumably baptized more people than Jesus, Paul may have converted more as a missionary, Moses led more Israelites out of bondage, Noah built a bigger ship, and Joseph Smith kept the Church together longer.
The point should be clear: if greater means quantity, there are many who fulfilled the Savior's promise that his followers would do "greater works," and this includes Joseph Smith.