It really depends on what you are looking at. This is a bit of an apples to oranges comparison, but OpenWrt happily works with 16MB of disk space, and can go down to 8MB if you squeeze it. It includes a modern Linux kernel, shell, networking stack, ssh server, package manager, text editor, web server with dynamic pages, etc...
Part of it's trick is that it aggressively pares down the hardware support, such that you normally download an OpenWrt image customized to your exact router. But of course the biggest difference is that it doesn't include a graphics stack or any GUI applications.
I work in embedded Linux, and its a whole different world here of trimming the fat on Linux to keep the BOM prices low. But you'd be surprised how lean we can get it.