iCloud Photo Library on multiple devices: how to keep a centralized location for originals?
When you shoot something with the iPhone, it is full sized only if 


It is recently clicked or opened. 
It has not been uploaded to iCloud. 


When iCloud gets the full size image, and iPhone gets low on storage, image is downsized.

When MacBook syncs the photos, it downloads the full size image and keeps it. 

When iPad gets the photo, I don't know what happens in transition, but ultimately, it has only a thumbnail.