Note that Emacs is still about 85 MiB larger than in NixOS 20.03. For a large part this is because the closure now contains two (!) copies of systemd, where previously it only depended on systemd.lib.

Edit: I created issue #99240 for that. We could just drop the systemd dependency in Emacs (looking at the source, it only affects running Emacs in daemon mode as a systemd unit, which I doubt anybody really cares about). However that wouldn't help since Emacs also depends on dbus, which in turn depends on systemd.

it only affects running Emacs in daemon mode as a systemd unit

And to add to this, it's only relevant for socket activation, which the NixOS service doesn't use.
But as you say, emacs depends on dbus anyway so it's probably not worth the extra effort, and it would be pretty nice to switch the emacs service over to use socket activation.