That would be so, as Skallagrim said, in their given universe. However, that's not ALWAYS the case in the human world. Larger people aren't inherently faster, for instance(See Big Show, Andre the Giant, and Great Khali). 

However, even in other literature, Humans do have a substantial advantage in that they are more versatile than many of the other races. Orcs are generally considered to be slower/dumber than humans, while Elves aren't as enduring as humans. That's why humans are popular, because they don't conform to any specific traits.