From following the directions for integration Cardboard into AR/VR sample (and relevant to making from scratch), if I apply the change Vuforia.VuforiaBehaviour.Instance.ApplyCorrectedProjectionMatrix(proj, eye == Cardboard.Eye.Left); as specified, I end up with small (short) eye cameras.
If I remove that code, the size is much better (but not perfect), but I have an issue where the virtual objects seems to "float" around when I move around my head (the camera) instead of being stationary on the target.
I looked up documentation and it specifically says NOT to use ApplyCorrectedProjectionMatrix for "see-through devices", which is defined as Google Cardboard. https://developer.vuforia.com/library/articles/Training/Vuforia-for-Digital-Eyewear.
Related to this issue here (looks the same):