This data, collected by @tomalmy and posted here, suggests otherwise:
One “gotcha” I haven’t seen talked about anywhere is the increase in size caused by the Universal binaries and, in the case of the OS itself, the added code to support Rosetta 2. In my measurements, Big Sur is 28% larger than Catalina. The updated, universal binary, apps have also grown. Pages up 26%. Third party app Affinity Photo up 62%. I experimented compiling “Hello World” in Xcode. Switching from Intel only to Universal it grew in size 245%! Since the App store apps are moving to universal binaries, even if you stay with Catalina expect to need more disk space.