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.

gamete is the ansewer