The Sonic that I tested (I have two but the other is unopened) is about as bright as an Arc AAA, though the beam is a little narrower. Other people's tests have shown similar results. It looks like CMG's regulator circuit wasn't as good, so the Sonic doesn't run as long on a battery, but you can buy a heck of a lot of AAA cells for the price difference between the lights.
Anyway, maybe your Sonic is below average, or maybe it needs a new battery.