To put what he said into perspective...

A twenty amp motor propped to draw twenty amps with a three cell li-pol (11.4 volt) battery is a 228 watt system. Using the above table that will power a one to two pound airplane depending on the level of performance you are looking for.
Keep in mind the weight you want to use is the figure your power requirements is that of both the airframe and power system (this includes the battery, ESC, motor and wiring).
(It all is kinda a vicious circle when you get into it.) To go back to the load concept the plane is the load for the prop and so on...

The Castle Creations website has a great power calculator application online that is a good resource as well.
Be advised: the phrase "Max Amps" typically refers to burst or peak power not the continuous rating; and this is were a lot of people get into problems with things cooking.