Medium DC motors generally have motor diameters between 40mm and 80mm. Contrast to smaller motors, these motors are more power and torque focused. They have bigger reduction gears to increase generated torque to a higher level which smaller sized motors cannot generate. They are generally used to work under heavier loads than their smaller counterparts. 42mm DC motors and 60mm DC motors are the most popular motors in this category.
Medium DC motors can both be used in hobby projects and industrial applications. Their big diameter allows the use of bigger reduction gears, therefore generating higher torque, but these motors are not used just for high torque. They can be used to work at high speeds, too.