Some items are not interesting for gameplay but they hold the power of transport you in a special atmosphere. LEM is one of them. Because of history, also because in one second the player now where he is. A mod, the LEM and you are no more on Valley but on the moon. It takes time to modelise this kind of item (in lowpoly for videogame). And the LEM file size is bigger than other ones, but we need it, so for a few items we can accept bigger files.
More informations about LEM : http://en.wikipedia.org/wiki/Apollo_Lunar_Module