Not sure if this is off-topic for this forum since the OS came out this year, but I just have to share.
I recently acquired an IBM ThinkPad model 365XD with its fully upgraded 40MB of RAM. I intended to use this as a development machine for some things I want to use in a classic computing club I'll be starting soon, so I wanted an OS which would support popular development tools like Git. NetBSD was one of the first things that came to mind because of its reputation for running on everything including your toaster, so I gave the documentation a look. It said you only need 4MB to run it, so I thought it was perfect and gave it a go on my Pentium III machine to test.
Well, I was slightly disappointed. The documentation was apparently out of date, because the generic kernel image was a whopping 19MB, and I was losing 21MB of available memory off the top immediately on boot. This wouldn't do. Some folks on IRC recommended building a custom kernel, and this seemed like a good idea. It was then that I discovered the GENERIC_TINY kernel config which was the one written for 4MB systems. Using this, the GENERIC config, and my dmesg log, I cut the kernel image down to a mere 5MB, and the memory claimed at boot is now less than 7MB, leaving my ThinkPad with almost 34MB to play with!
I could definitely squeeze more out by removing SCSI support (I left it in on the off chance I got a PCMCIA SCSI controller), but I'm quite pleased with the result. The only thing almost as fun as true classic computing is modern computing on classic hardware. :)