Does Armature current decrease as the load decreases?
For a shunt DC motor, the field and armature coils are typically connected in parallel, therefore the voltage is the same across the coils, and their current is proportional to their resistance. As you increase the voltage, the current will increase on both coils. So, to double the power (full load), the voltage and current have to be increased by the square root of 2 $$ 2^{1/2} x \ 2^{1/2} = 2$$
In the event that the coils are not connected in parallel and the voltage is to remain the same, then the current on the armature has to double, to provide the necessary power (ignoring losses).

It should now be obvious that the armature current decreases as the load decreases, and that for the second case at full load, the current would have to be 60A.