Holding torques of a 7W stepper and a 48W stepper
Pictures from here.

In short, the longer the stack-length the more torque can be produced per amp. So, if the basic model can take more current, it can generate more torque AND if its stack length is longer it can produce even more torque.

It's got nothing to do with the electrical power fed to the stepper motor when stalled - all that power is just wasted. After all, a bench vice doesn't consume power when gripping a motor armature to a standstill and, all the power fed to the stepper motor is turned to heat in a rotor lock situation.

Current and length are the primary factors for torque.