Looking at it really simply-

Think about how many teeth you're moving in the front versus how many teeth you're moving in the back (to make the wheel turn).

In the front the small ring is the easier ring (low gear). With one turn of the pedals you aren't moving that many teeth and will have to turn the pedals several time to move as many teeth as you would in the big ring. (But it will be easier to turn the pedals since you aren't accomplishing as much.)

In the back the big ring is the easier ring (low gear). As you're pedaling, the number of teeth you turn in the front will cause the same number of teeth to move in the back. If you're in the easier (larger) gear in the back, you'll have to pedal more times (move more teeth) to turn the wheel a whole revolution.

Hope that helps.