I don't know for sure how the implemented it, but I'm pretty sure that, only modifications are saved. There should be no need to save generated structures unless you modify them. If they were generated the first time you went through there, they will be generated the same way the next time you go through.

You should be able to walk over the entire world and, assuming you don't change anything, all that should be saved is the locations where you've been so that the map can be regenerated correctly.