My understanding is that if a light is brighter, it will light a larger area. (Like a point light.) If I try this with a volume light, it seems to only light an area proportional with its scale, not its brightness.
Here is an example:
Emission set to 10:
Emission set to 100:
Emission set to 1000:
The .blend I used to test this is here.
Is there a way to light a larger area without changing the scale of the volume?