[walteweiss]

So I have an old PC with i5 (Gen. 1, circa 2010 or 2011), and 4x 2 GB of ram. One 2 GB module broke and I hadn’t bothered to replace it for years, living with 6 GB. It runs Fedora Workstation 38, with Gnome. It runs it very well, and I came from more than a decade of MacBooks with 16 GB of ram. It runs it so well I don’t notice it’s just 6 GB in there.

[patrec]

I don't doubt you, but maybe your usage was never much memory constrained on either macOS or linux? What sort of software are you running? For what it's worth, I've got an even lower spec machine (4GiB), which works fine for light browsing and similar. But the minimum I need to run smoothly and simultaneously for a work machine is a browser (with plenty of tabs) and some mix of editors and IDEs. Linux craps for me with amounts of RAM that are way beyond what on a mac would still work fine. I don't think there's one single cause for that (e.g. safari is less of a resource hog than what's available on linux, zfs, which I happen to use, is better than apfs but probably also a bit more memory hungry, apple has virtual memory compression by default etc.).

Purely out of curiosity, what do

     ps -A -osize,cmd | awk '/[g]nome/{t+=$1}END{print t/1024^2 "GiB"}'

and awk '$1 == "Pss:" {t+=$2}END{print t*1024^-2 "GiB"}' $(ps -A -opid,cmd | awk '/[g]nome/{print "/proc/" $1 "/smaps"}')

print on your machine?