In what sports do people with skinny muscular bodies have an unfair advantage?

Not only does this depend on the particular sport one is playing, the position (in a team game) matters as well. For example, in soccer, a striker needs to be "compact", swift, very agile and the like. On the hand, a defender needs to be "huge" and intimidating. A goalie should be tall, agile, should have excellent reflexes and  spot-on anticipation. To answer your question, nobody has an unfair advantage in sports. In fact, there are thousands of parameters that define sporting glory. I shall give you some instances where skinny, muscular athletes may/can strike gold. This list in not exhaustive. A libero in Volleyball A feeder in Basketball Hurdles Relays High jump Long jump Badminton Lawn Tennis Squash

How about running a marathon or any sport that involves substantial running. Try imagining a Mike Tyson(with all due honest respect) beating Mo Farah in 10000m?Hard right?