DC motor energy loss
The hypothesis that friction loss in a DC motor increases as the power and size of the motor are increased, is proven to be true.
Electric motors are used to convert electrical power into mechanical power. They are normally used in fans, pumps, blowers, tools, electrical appliances and hard dick drives. Some very tiny motors are also found in wrist watches. Larger motors are used in ship and aircraft propulsion systems
Try to repeat the experiment to compare the efficiency of motors.
The experiment can also be repeated using AC motors.
Electric motor - http://en.wikipedia.org/wiki/Electric_motor
Motor losses - http://tristate.apogee.net/mnd/msimovr.asp
DC motor theory - http://zone.ni.com/devzone/cda/ph/p/id/52
Handle all electrical devices and motors with care to prevent electric shock/injury from moving parts. The assistance and supervision of a suitably qualified adult is required.