The same amount of power is going to your speakers, large or small, it's just that there is more power reserved for the "big bang" stuff. (I think I have that right)
When the Mains are "small", the crossover frequency set in the AVR will limit the low frequency of the mains (Set the crossover to 80Hz, and the mains start rolling off below 80Hz). So, in effect, the mains do get less total power since they are not reproducing the frequencies below 80Hz (they will output the same power above the crossover frequency however). The sub will pick up the missing frequencies, so for Mains+Sub the total power is the same.
