Adults have much larger muscle mass than children. You reach a peak of muscle mass at about 25ish (my age!). After this it declines (sad face!). So your muscles are still growing all the time.
They get smaller again through loads of different processes in our body, a lot of scientists actually don’t agree on the reason for this and we don’t fully know how or why it happens!
Some say its because of disuse, so if you don’t use it you lose it! Or it could be inflammation in our muscles. Hopefully our research will answer this!