to be fair, i think it depends what that graph is measuring.  if it is
mainly more device drivers and protocols, that increases the scale of
linux, but not necessarily the complexity.  whether some of those
protocols and driver functions really need to be in the kernel might
of course be relevant.  the complexity also isn't a function of size,
but of construction (as a small example i think of all those #ifdefs
in the Linux kernel code i've seen, particularly the powerpc and arm
support).

on the other hand, i think rob observed once years ago at a conference that at
the time the entire source code of the Plan 9 kernel was smaller than
just the include files of some Unix systems, and i don't think that has
changed significantly.