It isn't just about tightness or clamping force although that can be a reason for ejection.

My experience is that water bottles eject when the lip at the top of the cage doesn't mate with the groove in the bottle correctly. This isn't the only reason but one of the main reasons.

Bad...



Good...



The higher quality bottle in the bottom picture takes significantly more effort to remove from the cage and "clicks" into the cage when inserted. The bottle in the top pic isn't even held by the cage. These are high end FSA Speed Cages pictured.

If you want clamping force then get X-Lab Gorilla or Gorilla XT cages. Together with properly shaped bottles you should never have an ejection again.
-Tim-