system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If muscles are built by working out and making small tears in them then getting rebuilt, why do they grow smaller with lack of exercise?
gpt
It's harder to imagine in our society, but in almost all of nature *calories* are the currency. Top priority is to conserve them.

What's the point in having large muscles that you never use? They're wasting your calories supporting themselves, and providing you *nothing*. 

To be beneficial in evolutionary terms, the muscles need to help you do something. Otherwise it's more beneficial to scrap them. So your body builds just the amount of muscle that you need to do what you do.