There's also the fact that the lower the resistance, the greater the amp draw, leading to far shorter battery life between charges. But "underpowered" would seem to indicate some fairly major voltage drop, which I know is a factor with some mechs, and perhaps even some APVs -- my Vamo seems considerably underpowered compared to my sigelei zmax.
But I'd have to second the opinion that if you're asking questions about coil *size* you perhaps need to study up on Ohm's law a bit before using a mech, and certainly before doing anything drastic with it (such as sub-ohm vaping -- very drastic!).
Andria