The operating system will page out pages that have never been used, or used very infrequently, if at all.
 
If you have a 6MB program, and there is 500K of code that's used on startup, but never referenced again, let the operating system page it out, and use that RAM for other programs, or cache. The same goes for code that implements functionality that is rarely (if ever) used. Why load it, if it never gets referenced?
 
In a regular program, there are lots of pages that are referenced once, or not at all.