A servo motor can be any type of motor that has a control and feedback system that provides precise precise control of angular or linear position, velocity and acceleration of a driven load. For a valid comparison of speed and torque operation range vs. motor mass and volume, it is necessary to consider specific types of servo motors. Feedback devices and their requirements must also be considered.
There are also more than one type of stepper motors. Specific types need to be considered for a valid comparison.
Motor power is torque multiplied by speed. Motor torque is proportional to motor volume. Motor speed is less closely related to motor volume. To some extent, a motor of a given mass and volume can be designed for a specific torque and a wide range of speeds. In this case, the power is determined by the selected speed.
The continuous torque capability is related to the method of removing heat due to losses in the motor. Most motors have a means for the motion of the rotor to move air through the interior and/or over the outer surface of the motor. The physical and design of the motor influence the heat removal capability and thus the continuous torque capability. Higher rotor speed generally means more effective heat removal and higher continuous torque capability.
Not every servo motor is designed to provide continuous, or long term holding torque. Providing continuous torque with no rotor motion may require special cooling provisions or a physically larger motor.