In the brushless motor family the outrunners make the most torque. However you have to use the right one for the aplication. Because the torque comes mainly from the diameter of the motor the secondary feature that modifies the torque capability is magnet length. That means in a restricted space you want a longer OR motor for a given diameter to acheive higher torque. In addition you must reach a target RPM so then you factor in gearing and KV.
If your seeing low torque for this motor then voltage will help as long as the Amps are not too high. At low RPM with a high load you need more RPM from lower gearing to overcome the load.
Maybe this part of the set up needs to be perfected a little more.