Answer
A harp is not as loud as a piano because its sound board is smaller and does not move as much air.
Work Step by Step
Both stringed instruments rely on a sound board, which is a surface that the strings vibrate against, and set into motion. The sound is made louder because the the vibrating sound board has a large surface, and moves a lot more air than the vibrating strings can move on their own.
This is discussed on page 382.
