You're asking about illuminance (how intense is the light arriving at a surface) as a function of distance AND size (scale) of light source, keeping all other things equal.

The lighting equation has many terms but we'll use a simplified one where the variables are roughly just the ones you were changing.

Letting I stand for illuminance, B stand for the brightness of the light source, R is the distance to a light source, and S is the size (scale) of that light source we have

I = B (S^2) / (R^2)

In English, the illuminance is proportional to the brightness of the light, the square of the scale, and inversely proportional to the square of the distance.

Your first case you claim the distance is 10. But the coordinates you gave (z10 y10) would make the distance roughly 14. As well you didn't mention an x offset but I can clearly see that the light isn't lined up on the Z axis so I'll assume x10 as well. Therefore the distance squared (R^2) is 10^2 + 10^2 + 10^2 or 300.

I = B (100^2) / 300 = 33.3 B

Your second case we have z1000 y1000 and I'll assume x is still 10 (looks that way based on the pattern of specular highlights and shadows). This gives a distance squared (R^2) of 1000^2 + 1000^2 + 10^2 or 2000100. You also said the scale is now 10000.

I = B (10000^2) / 2000100 = 50 B

50 is bigger than 33.3, therefore we expect the figure to be brighter.

However, I didn't verify my assumptions about the lighting terms in the version of Poser you're running. There COULD be a bug or other factor I didn't take into account.

For example, I'm not absolutely sure the brightness of the light source is unaffected by changing its scale. If it is, then I'm missing some of the correct equation.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)