Most likely since the vertexes are spread out (by enlarging) it takes less computing to figure out where a surface is and if it interacts with another surface.  But that is just a guess.  Also the scale between the background and the object are closer together.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13