I do not know about Poser internals, but other (older) software used octree culling, which is a binary search in 3d, in each step it examines all the cubes, and tosses out all with nothing in them.  a small object takes more levels to find which cube(s) the object occupies 

https://en.wikipedia.org/wiki/Octree

Bill

People that know everything by definition can not learn anything