Why are large earthquakes more common than small ones ?

They are more common because it takes a years for the stress to build up and the earthquakes to be huge. 

Each level on the Ritcher scale adds 10 times the force of the last one, so the higher you go, the harder it gets to rise. Another reason for small earthquakes is friction. In order for an earthquake to occur, friction has to buil up between the two plates as they try to pass, massive blockades are needed to hold back to forces of the shifting plates. When the forces overpower the friction, the plates break past each other. There are less areas where that kind of friction exists, so smaller earthquakes happen far more often.