As was mentioned, these guys really aren't big. They are just so very lean that they apear larger than they really are. The goal of a sprinter isn't muscle mass which ultimately equates to a higher body weight. The lighter the load the easier it is to propel it. One thing to keep in mind about sprinters is that they train anaerobically...Both through thier sprint training and weight traing, so this has quite an impact on thier muscularity. Sprinters, contrary to popular beleife, aren't endurance atheletes hence they actualy have muscle because they need it.