Size and complexity
Upstart (1.5): 285 files, ~185k lines, ~97k C
Debian: sysvinit + 120 files, 5.8k lines
systemd (v44+): dbus + glib + 900 files, 224k lines, 125k C
sysvinit: 560kB, 75 files, ~15k lines
Debian startup is smallest, it's only shell with sysvinit (C) as dependency
Upstart is about 10 times bigger in terms of lines of code/text. Most of the extra complexity size comes from C.
systemd is about 10 times bigger, like upstart. But with the mandatory deps it blows up to about one hundred times the code footprint! Most of the extra code is in mandatory dependencies, but the systemd core is also bigger than anything else.