Bryce atmospheres are very dependant on scene size (dimension). The density of the fog is based entirely on unit size, if the scene's objects cover more units, then there can be a greater difference in 'fogginess', depending on how close the objects are to the camera.
If you select everything, group it, and scale it up to larger size, then the atmosphere can have more depth. The near objects can be barely fogged out, but the far away objects can still be heavily enshrouded.
Don't worry about grouping, Bryce does sub grouping very well.

EDIT - Put the camera in the group before rescaling. Saves you from having to move it.

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