Vertex shaders gets called a lot less than fragment shaders. It's because the fragments get generated from the vertices (by interpolation). Let's say that you have a triangle `ABC` like this: 

    a = 0,0,0 
    b = 1,0,0 
    c = 1,1,0

Depending on the resolution, this generates some amount of fragments covering the whole triangle.

With certain projection configuration this will produce a triangle which covers half of the screen, which at 1920x1200 resolution and back faces culled will be `1920*1200/2` fragments. Without back face culling, it will be twice that. The amount of fragments increase with every triangle you draw.

[A fragment is not a pixel][1]. It is much more. 

If vertices send some properties to the fragments (like a colour), they get interpolated across the surface of the triangle to all the fragments.


  [1]: http://en.wikipedia.org/wiki/Shader#Simplified_graphic_processing_unit_pipeline