But even assuming the high end of your estimate, Postgres + PostGIS would still be less than 1/10 of the size of a baseline Oracle installation.

660 mb disk space?

I can afford that.

And I think that most people can.

For the record, on Debian unstable:

  root@pg1:~# LANG=C apt install postgresql-12 
  Reading package lists... Done
  [...]
  0 upgraded, 32 newly installed, 0 to remove and 0 not upgraded.
  Need to get 59.7 MB of archives.
  After this operation, 240 MB of additional disk space will be used.
  Do you want to continue? [Y/n] ^C
  root@pg1:~# LANG=C apt install postgresql-12 postgresql-12-postgis-3
  Reading package lists... Done
  [...]
  0 upgraded, 105 newly installed, 0 to remove and 0 not upgraded.
  Need to get 104 MB of archives.
  After this operation, 418 MB of additional disk space will be used.
  Do you want to continue? [Y/n]

I think the addition of LLVM for JIT execution of plans added a lot to the Postgres install baseline (at least in distribution packages, you can compile a much leaner postgres yourself).

Due to the way that the LLVM dependency in postgres works, it could be packaged separately from the base postgres server package. The LLVM interfacing code is runtime loaded, and there's no error if "llvmjit.so" is not on the system, even when the server was built with LLVM support. It's basically a packager's choice whether to do so, or not.