The explanation of the differences in the performance of stepper motors and servo motors in a very brief summary:
The whole secret lies in the overload capacity. A stepper motor has a torque dependent on the current which is flowing through the windings. This current is inflicted by the driver. If the torque is exceeded, the engine "loses steps" and the driver does not know about it. As a result the machine goes further and destroys the material. To prevent such occurence the stepper motor must be loaded with the torque not exceeding 50% of the rated torque at given rotations.
The servo motor works differently. It frequently analyses the position offset, i.e. the difference between the position given and the current position (e.g. from measuring linear scales). The microprocessor in the driver tries to control the torque, that is the motor current, so that the position offset is as small as possible. Due to the fact that the current overload capacity of servo motors reaches a tenfold nominal value, the perceptible power of the machine equipped with servo drives and of the size similar to stepper motors is many times greater.