FreeBSD, NetBSD, and OpenBSD Installation Size Comparison!
Below are install sizes measured today by du -sh / for FreeBSD, NetBSD, and OpenBSD KVM VPS installations.
These installations each are approximately the default installs, except that all three are fully self-compiled. Each install contains the -current source code for the base system, the X Window System, and the -current install framework for optional packages a/k/a ports.
Each install also contains the object directories and files from its most recent compiles.
FreeBSD and OpenBSD default installs use the LLVM compiler. NetBSD uses GCC.
Not many ports are installed, mainly just tmux (except, if I remember right, on OpenBSD, where tmux is part of the base system).
In addition to tmux, the FreeBSD system has the git-tiny port installed (rust and a second, additional LLVM version are installed as dependencies in FreeBSD's default git port).
The FreeBSD install is running on UFS, like the other BSDs. Compression from ZFS, if ZFS were in use, might decrease the FreeBSD install size significantly.
Each system also contains a few extra files -- about 50 MB on FreeBSD, maybe 25 MB each on Net and Open.
freebsd:~ # du -sh /
40G /
freebsd:~ #
netbsd# du -sh /
35G /
netbsd#
By default, NetBSD pkgsrc saves downloaded current and previous versions of upstream package distribution source tarball files ("distfiles") locally.
netbsd# pwd
/usr/pkgsrc
netbsd# du -sh distfiles/
7.5G distfiles/
netbsd#
The NetBSD install has been around more than a year, and there are multiple versions of multiple distfiles, maybe including some that are not currently installed. So, let's arbitrarily adjust the NetBSD install size from 35G to 27.5G.
openbsd# du -sh /
10.7G /
openbsd#
I expected the adjusted NetBSD default 27.5G install + compile size to be the smallest. I was surprised that the OpenBSD default + compile size of 10.7G measured only a little more than a third of NetBSD's 29.5G adjusted default install size!
Of course, all of these installs could be smaller if optimized for minimum size.
Thanks to @linveo for sponsoring the VPS on which NetBSD is installed! 
I hope everyone gets the servers they want!