hi Mick,
As an example, say you wanted a resistor to be heated so that it dissipates 100Watts.
You could have a voltage supply of 100V and the current would have to be 1Amp
If you have a current source that has only a 0.5A output, you would have to increase the voltage to 200V in order to maintain the 100Watts.
The point you're over looking is the phrase, "For a given wattage".
For a fixed amount of watts, the current needed will be less if the applied voltage is increased.
Of course, this demands that the resistance of the circuit be larger to support the increased voltage at the lower current, but this isn't (usually) how you design circuits. This is how you check your design to see if it will smoke. You learn Watt's Law and then set it aside until you need it to design a heater or double check a voltage design.