What's Armory's RAM usage? I think you are confusing RAM used by the process and in use memory pages. The OS won't shy away from filling that up when a lot of access is performed on mmap'ed files, which will happen a lot during a DB scan. That doesn't mean the RAM is in use, it means the OS is chugging unused RAM to shuffle file data in and out of it. If a process was to ask for more RAM it would get priority over this memory.
The OS has no reason to empty this memory once Armory is done scanning. For all it knows, Armory just asked for intensive access to this data, so the OS expects it will again. Unless other processes start asking for more RAM or Armory is closed, the OS will keep these unused pages filled with the DB's mmap'ed files.