..Think of it this way...Who has the more lean muscle mass. Sprinters or long-distance runners? Sprinters...Long-distance runners appear thin, but have a much higher fat content due to running at higher intensities for prolonged period of times. Sprinters have lean healthy muscle mass and don't waste their muscle stores.
Actually (elite) marathoners spend the bulk of their running time at what would be considered low-med intensity. Their muscles are smaller, true, but that is because they do endurance weight training (high rep, low weight) if they weight train - because building muscle mass beyond a certain point is detrimental to endurance running.
Sprinters have bigger muscles because they spend the time in the gym building them, because the power of explosive muscle is worth the weight for short distances. And they spend far greater time in the high intensity zones than marathoners.