The root filesystem should generally be small, since it contains very critical files and a small, infrequently modified filesystem has a better chance of not getting corrupted. A corrupted root filesystem will generally mean that the system becomes unbootable except with special measures (e.g., from a floppy), so you don't want to risk it.
The root directory generally doesn't contain any files, except perhaps the standard boot image for the system, usually called /vmlinuz. All other files are in subdirectories in the root filesystems:
