Austin and I were running some numbers on the KSBD demo area DSF as part of a discussion of how instancing will someday allow X-Plane to render more objects. (Instancing is the ability to render multiple simple objects with a single instruction to the GPU...the requirement of one command to the GPU per object means that total object count is bottlenecked by the CPU->GPU connection.)
Here's some numbers:
KSBD contains 868,220 mesh vertices - at 32 bytes per vertex, we have about 27 MB of geometry per DSF. In one view we picked, about 20% of those vertices were on screen. (But since there are six DSFs loaded, really only about 3% of a DSF mesh is seen at one time at lower altitudes.)
KSBD contains 153,816 objects. Near the airport, we average about 238 vertices per object. (This is a GOOD number - less than 100 vertices would imply that we aren't sending out enough vertices for each command to draw.) But this means that if we were to simply store all of the objects as one huge object, we would have 1.1 GB of object memory! This is why you can't just make a single huge object for the world. :-)
(X-Plane of course stores each OBJ file once, saving a lot of memory. But then we burn CPU power telling each OBJ to be drawn over and over in many places.)
It also explains why forests keep causing people to run out of memory. Consider that only about 3% of the mesh may be visible (when we're at lower altitudes, where you can see the trees). This means that we need about 30x as much memory to storage geometry as the card can draw. With cards so fast, we easily run out of memory before we max the card out.