Just guessing here but perhaps the wick would need to be more saturated to fill the volume of the larger bore coil? Also, perhaps a smaller bore coil with more coils would give more surface heat to a thinner wick possibly producing more vapor?
I have been much happier since running 1/16 I.D. Coils with more wraps to meter at similar ohms.
Shot thru cyberspace with my iGun