What you're seeing is called quantization noise, and is the result of applying a bump map that does not have enough dynamic range.

Bump maps, being 8-bit images (generally) can only take on 1 of 256 distinct values. You may think you're seeing continuous variation in the surface, but this is not the case. The surface bump level is always a stair-step function, with the smallest possible step being 1/256th of whatever your maximum bump value is.

The "contour" lines you see are the edges of on of these levels. The lines are showing wherever there is a "step", for example from 141 to 142.

The reason you see them when you render big is that texture filtering at that size is not needed. So Poser is rendering the bump map exactly, which creates the steps.

When you render at normal sizes, texture filtering applies, and it is interpolating and averaging those values, thus creating the appearance of numerical values BETWEEN the steps, such as 141.23245.

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