Hi. Thanks for your query and for using Health Care Magic
Its difficult to comprehend what exactly shrinkage means.
I medical terms in Chest X Ray
, we use a terminology such as "Volume Loss". Your brother seems to having Volume Loss on the Right Lung.
There are many causes for such condition.
1. Leakage of air around the lung can compress the lung and push it inwards - Pneumothorax
2. One out the three lobes of the lung can undergo collapse as this can lead to volume loss on the Right Lung - Collapse
3. Some people have birth defects such that some part of lungs are under developed. This shows up in X ray as volume loss
You need to find out the diagnosis for the Volume loss
Do get back once you have more information