So what you are after is larger disk of light in the light shining through your mask?
I don't know if that's even possible in a 3D package when doing volumetric lights (I've just never tried it with a parallel light source before).
But in Shine it all comes back to a single point - same goes for the radial blur set to zoom if were you thinking of trying that.
But you play with the boost and ray length settings you can reduce the idea that it's all pointing back to one source by making the beams shorter but brighter. When the eye has less of a line to trace back to a source it's harder to see where that source started.
Another way is to start with your mask and fill it with a glowing blob. Precomp that on black and then put shine on the precomp. This way its the non-zero point blob that is making the light so the brightness will seem spread over a larger area.