Why 9+ quakes are rare?

 Today early morning, between 5 am to 6 am,while doing meditation, I had an idea (queation) as to why 9+ quakes are rare.

Actually, whole concept of Earthquake prediction was born out of morning meditation.,including recent concept of -Earthquake epicenter hypothesis-
I knew the answer given by plate tectonics in general and USGS in perticular,but some how I am not convinced .
As per USGS more the length ( and in turn contact area) at plate boundaries/fault line ,bigger will be the quake.
  Now ,as we ( ie plate tectonics) have already concluded that ,the built up stresses at plate boundaries ( when they are suddenly released)are the only  cause  of Earthquakes.
The built up stresses are measured per unit area.and it has nothing to do with total contact area. On the contrary lesser the length of plate boundaries greater the stress per unit area.
It also implies that, more the depth of plate boundaries, more the area of contact ,and lower stress per unit area. Therefore shallow plates ( ocean plates) should bave always smaller quakes. All near 9 quakes should occur at continental plate boundaries
The topic is open for discussion