I had a reactor on my 90G and I recently switched to a 45mm GLA atomic diffuser. I'm using about 1/3 the CO2 (BPS) with the diffuser compared o with the reactor, and i'm getting the same concentration level in my aquarium.
As Jeff5614 said, your comparison is not even remotely valid. Reactors have extremely low working pressure. Atomic diffusers have very high (by aquarium standards) working pressure. That means the each bubble with the diffuser might actually contain, like, 9 times more CO2 than each bubble with the reactor. In such example, if you are running your diffuser at 1/3 BPS, you are actually using 3 time more
CO2 than you did before. More, not less.