If Jay loses muscle size after taking a break, this loss is called .

If Jay loses muscle size after taking a break, this loss is called atrophy.  

Explanation:

Muscle atrophy refers to the condition in which the mass or size of the muscle gets reduced to temporary disabling of routine work. This may lead to either partial or complete damage to the affected muscles.

On the other hand, hypertrophy refers to the condition in which the muscle size gets increased wither due to over storage of glycogen or increased size of myofibrils.  

As Jay loses muscle size after taking break, it may be inferred that he is affected with muscle atrophy. hope this helps you :)