Jesus taught them what big things they could accomplish if they had true faith.

Luke 17:5-6

5 One day the apostles said to the Lord, “Help us to trust in you (OR, in God) more strongly!” 6 The Lord replied, “Mustard seeds are very small, but in this area they grow and produce large [MET] plants. Similarly, if your faith grows until you truly believe that God will do what you ask him to, you will be able to do anything. You could even say to this mulberry tree, ‘Pull yourself out with your roots and plant yourself in the sea!’ and it would obey you!”