Maybe I don't explain so good, so try again.

Scaling is for the light indicator only!

for honest I don't know why they pick 55% for default scale on infinite lights.
But think, you can't scale anything infinite anyway, the circle for infinite distance lights (of which IBL is member) is to help you plan.  The physical origin act like infinite distance away.

If you look on page 201 of Poser 8 manual it tell you not affecting light.

If you save scene file with scaled lights, they reset to default scale when you reload, so is only ment for temporary use.