I'm not thrilled about how big the early 4.15 rc's are, but rc3 is often the biggest rc because it's still fairly early in the calming-down period, and yet people have had some time to start finding problems. That said, this rc3 is big even by rc3 standards. Not good.
Most of the changes by far are drivers (with a big chunk of it being just syntactic changes for some doc warnings) with some perf tooling updates also being noticeable. But there are changes all over: core kernel and networking, kvm, arch updates and Documentation.
Anyway, I sincerely hope that things are really starting to calm down now.
Also, there's a known issue with x86 32-bit suspend/resume that I just didn't get a good patch for in time for this rc. Soon.