Streaming pool size is used by textures, so nothing to do with Nanite.
If you want to keep it down, you need to use smaller, or less, textures.
Bear in mind, that the streaming pool isn’t managed in editor, only in builds. So the longer you spend auditioning textures ( assets ), the fuller it gets.
If you want to know for sure if your level is a problem, restart the editor, and only open that level.