I was on University of Arkansas track team in 1991 and 1992. And yes they have won over 30 National Titles.
True alot of this is due to genetics. But I would attribute it to the types of workouts.
The Long distance runners obviously have no muscle. This is due to extensive overtraining.
The sprinters however do a completely different type of training. The only distance running they do. Is maybe a half of mile to a mile just for a warm up. The rest is bounding, sprinting, lifting, ect... These excersises use fast twitch muscle fibers and not the slow twich which the distance runners use. I believe these excersises cause for muslce growth and not loss.
However, I will say this. Just like bodybuilding - it is an illusion. The sprinters actually look a hell of alot bigger than they really are due to the extremely low body fat. There arms may only be 14 to 15 inches but due to no body fat, they look like 18" arms from a distance. I competed with many atheletes which weighed 140 to 150 lbs. then saw them on a highlight show and thought damn he is fucking big.