I'd be curious to see services designed to run as PID 1 inside containers, and contain or run nothing else other than the required binaries. Maybe someone is doing this.
It's pretty common for languages that compile to binaries. Golang, for example. Just inherit your container "FROM scratch". I have a number of service containers that are around 8 - 12MB, which contain compiled Haskell binaries.
Pretty sure this is what tini[1] is for, and there's supervisord[2] for more complex use cases, e.g. running multiple processes. Neither are a full replacement for traditional systemd, which I heard you can technically run in a container these days, but I've never seen anyone try.