Let me put it this way: There are 15s rated at 125W RMS, there are 8s rated at 125W RMS. That means that both diameters require 125W RMS to fully accomplish their purpose.
but you will never find the same MODEL subwoofer (with the same technology, etc.) with the 8" and the 15" with the same power handling... or at least i have never seen one... sure with different models the power handling changes, but as a whole, if you take the 8" and the 15" of the line of subs, be it the L7, W0, W3V2, whatever model it is, the 8" will always handle less power than the 15"... what are we arguing about? we both agree on the same thing 
However, an 8" will not be able to reproduce the same frecuencies the 15" does.
right. that's been established...
Lower frecuencies move greater volumes of air. That is what requires the power, not the size for itself. Size makes possible to move those amounts of air in a better way.
it's all the same thing!
Large Sub = Lower Frequencies = Larger Cone = More Power to move the cone.
you see? they're all connected. but the reason a large sub requires more power is because it's got a large cone. just like a large lightbulb requires more power than a small lightbulb, and a large car requires a larger engine to move than a small car. we agree on this, we're just mentioning different aspects
If size was the determinant factor for power handing, there would be no point in making an 8" rated at 50 RMS and another rated at 125, or a 15 rated at 150 and another rated at 1000.
but they're different models... you can't expect the same from the W0 range as you would from RE's XXX range... they're using different technology, etc. so OBVIOUSLY not every 8" sub in the world's going to have the exact same power handling
More RMS will only make a sub to reproduce the same frecuencies louder than another of the same size with less power handling, assuming they are similar in all other aspects.
what are we arguing about?
