nLite was popular in the carputer scene. I ran a stripped down XP version on a low power industrial PC (Atom CPU, 1GB RAM, laptop disk) hooked up to a Lilliput touchscreen up front.
I credit nLite with improving my understanding of Windows. I was educational to see all the components listed with tooltips about their function and dependencies. It was like building your own custom copy of Windows. At one point I think I had a copy of XP running using 50MB of RAM at the desktop.
gentoo (and begrudgingly, i'll admit ubuntu server) are really light. I run web app servers on mostly gentoo, ubuntu if it requires docker, and a base install is 30-50MB of ram used. Right now my cgi, syncthing, and web fileserver is using 125MB. misskey server is using 725, i think it uses 175 on boot, so possible memory leak there. matrix server is using 363MB. Mattermost server been up for 3 months and is using 383MB.
if you remove all the source files and cruft from a gentoo install when you're done setting it up, the actual system is less than 1GB on disk, and as mentioned, 30-50MB of memory used. Ubuntu is similar, although it does include more helper apps so the disk weight is higher.
definitely installed xp netbook edition, and it was faster than full boat xp but still a chonker compared to whatever the popular linux distro for netbooks was back then