Different people have different ideas about what a small, functional Linux installation might involve.
In some cases, application selection can lead to enormous savings.
Xfce instead of GNOME or KDE.
Mutt instead of a graphical mailer.
Links and/or Dillo instead of Mozilla.
Less featureful X server and configuration.
Consistent choices like this can get a machine down to running happily in 24 MB or 32 MB of RAM on a slow processor.
One person's idea of functional might be unusable to another.
All of these systems have different requirements:
One disk firewall or router.
Forensics disk.
Linux Video Recorder.
Developer desktop on low memory, low diskspace i486.
There are trade-offs between disk usage, memory usage, resposiveness and boot time.