The best word to describe small DC motors is versatility. These motors provide a great variety of torque, speed and weight options. Motors which have a motor diameter between 20mm and 40mm are generally classified as small DC motors. These motors almost always have reduction gears to increase the generated torque. 25mm DC motors and 37mm DC motors are the most popular motors in this category.
Small DC motors can be powerful enough to be used in Sumo robots with a higher reduction ratio and can be fast enough to be used in a line tracking robot with a lower reduction ratio. These motors are seen quite often in robotics projects because of this high versatility in operation.