The main systemd binary (version 208 for x86_64 as shipped by Arch Linux) is 1020 KB. I get that the systemd package is really several binaries, but still, does the binary for PID 1 need to have that much in it?
The problem is that software complexity grows exponentially with code size. Well, maybe not exponentially, but more than linearly. And the article established that it's important to keep PID 1 simple.