From what I have read, the distance that the engine uses to dicide when to render object depends on LodRatio, ViewDistRatio, and object size.Quote:
mole1, the size of the model is not what lets you see it from further, the vewdistance is.
Size make sence, simply because the further an object is, the small it gets. A shell at 1000m is a speck and will not be drawn, while a mountain at 1000m will look like a small hill.
The engine does strange things, apperently it looks at objects in Ranges ( lodratio) if the object falls within the range to be drawn it is drawn.
Or at least that is how I understand it, Documentation on the engine is more than lacking