modern 2nd stages typically use a reasonably large amount of venturi effect, and so when you inhale, the reg is (to some degree) actually blowing air into your lungs, ie it is supplying air at a total (static + dynamic) pressure above ambient water pressure. When you exhale however, there is no such effect, and your body has to do the full amount of breathing work, and in fact, because you are exhaling through the exhaust flapper valve, there is an additional loss not present during inhalation.
Look at a typical work of breathing graph:
and notice how skewed towards exhalation it is, and how during inhalation the venturi effect actually crosses the zero line