Some distributions have been constructed with an eye to building things like routers and firewalls. By keeping them small and simple, they are easier to make and keep secure.
This is a small Linux distribution that integrates various security tools.
A fully-functional Linux distribution that fits into 16 MB. Many modern net-appliance-type products such as the Netpliance iOpener have an internal SanDisk device that is 16 MB in size.