I like the notes in the NET Bible on John 14.12.
What are the greater deeds that Jesus speaks of, and how is this related to his going to the Father? It is clear from both John 7:39 and 16:7 that the Holy Spirit will not come until Jesus has departed. After Pentecost and the coming of the Spirit to indwell believers in a permanent relationship, believers would be empowered to perform even greater deeds than those Jesus did during his earthly ministry. When the early chapters of Acts are examined, it is clear that, from a numerical standpoint, the deeds of Peter and the other Apostles surpassed those of Jesus in a single day (the day of Pentecost). On that day more were added to the church than had become followers of Jesus during the entire three years of his earthly ministry. And the message went forth not just in Judea, Samaria, and Galilee, but to the farthest parts of the known world. This understanding of what Jesus meant by “greater deeds” is more probable than a reference to “more spectacular miracles.” Certainly miraculous deeds were performed by the apostles as recounted in Acts, but these do not appear to have surpassed the works of Jesus himself in either degree or number.
and the Bible Knowledge Commentary...
The apostles would not necessarily do more stupendous miracles than Jesus did (e.g., feeding 5,000) but their outreach would be greater (e.g., Peter in one sermon had 3,000 converts). This was possible because Jesus had gone to the Father and had sent the Spirit. Miracles are important, but some evangelists have done even greater things than these by preaching the good news to many thousands of people.