The kernel code is growing in an exact proportion to the amount it contains within the kernel
So, for example, the core kernel makes up 5% of the overall size.
And exactly 5% of the changes happen in the core kernel code itself.
Same goes for every other category of the kernel code, it's pretty amazing.
I have a script that digs all of these numbers out of the changelogs on my kernel.org
directory for these kinds of statistics.