Occlusion confusion - could use a quick explanation!

Hey all, I was searching around the forums but could not seem to find a decent explanation for what I am seeing. Its probably not even a problem - just me failing to understand the issue.

I am trying to understand occlusion culling so I started with a terrain and some cubes. I set the cubes one behind the other (so that the largest is in front and a super tiny one is in back. I make sure to set everything to Occluder/occlude static and bake. Good so far.

Now here's my confusion - when I test this with my cubes it only slightly works. When I move super close up to the largest cube in front with my camera, then everything behind it occludes perfectly fine - so it is working some what! But once I back the camera up just a little bit - nothing occludes behind? Its not like the cubes(which are super shrunk) behind it are anywhere NEAR being seen either, despite me backing the camera up a bit. Am I just missing something really obvious here?

 

Also attached are just 2 simple pictures to better show what I mean. The first image is me very close to the cube, the second is me a bit further out. Again to clarify, I am no where NEAR seeing the smaller cubes in the back at this distance.