Why do people who participate in marathons tend to have smaller muscles?

Because they more focus on muscle endurance then that of muscle strength.

Explanation:

Long distance runners need their bodies to better focus on their muscles to all work together at the same time to help them all keep a steady pace, go a long distance and more. Muscle strength is more focused on individual muscles themselves, have one or a few used at once. That is to build up certain muscles that will help with other things, not long distance running.

well i’m not to well in health but if the body produces more standard fat then the fat will convert most of it into energy and give all the muscles nutrients (sorry if it was wrong  

i believe is was