http://en.wikipedia.org/wiki/Mechanical_advantage

Power (how fast you're going) is always going to be equal to torque (how hard you push the pedals) X cadence (how fast you spin) no matter what gear you're in. When you shift into an easier gear, you don't need to push as hard but you have to spin faster to maintain your speed. One revolution of the crank is going to make the back wheel spin a shorter distance then when you were in the harder gear.