Although I do a clean, fresh installation of the Mac OS, that article is somewhat scary. And I've noticed that with Big Sur, the total space taken up by the OS and my apps, settings, files, folders, etc. is about 35 to 40% higher with Big Sur that it is with Catalina. For both of my Macs, I always reserve the entire internal SSD (256 gig on my Mini, and 252 gig on my MacBook Air) for the OS and those other files I mentioned. I also make a concerted effort to keep both of my machines "lean, mean, and clean", and thus have never had the issue referred to in that link.
I did, though, very recently have an issue with space and Big Sur, but it was while attempting to create the eDrive used in Tech Tool Pro. Using the version of Disk Utility that came with Catalina to Erase and Format a drive that already had Big Sur on it (was having some application-related issues), when I attempted to create the EDrive (only 33 gig in size), got an error message of not enough space available (issue has to do with the creation and management of containers by the OS). But when I used the version of Disk Utility that comes with Big Sur to do the Erase and Format, that issue went away.
For some "within the same OS" upgrades, I will just download the Combo Updater and apply it. Usually not much of a difference in space used.
