Because the higher the voltage applied on a motor is, the higher the magnetic field intensity is. If the magnetic field is too strong for your core (the rotor in the case of a motor), you end up in saturation territory which usually results in your motor getting destroyed. In order to increase the capacity of your motor to handle bigger magnetic fields, you need to increase the size of the rotor which result in a bigger and heavier motor. The same goes with transformer by the way. So to increase the voltage, you must increase the motor size. See https://en.wikipedia.org/wiki/Saturation_(magnetic)