It seems that the developers probably missed out on sending their pull requests for rc2 which may have led to higher number of changes in rc3 leaving the third release candidate to be “much bigger” than its predecessor. The reason for Torvalds’ dissatisfaction is apparent because as the Kernel development cycle progresses, release candidates tend to become smaller.
The rc3 doesn’t have anything scary in particular but, Linus isn’t too happy with the way things have churned out. Torvalds has revealed in the Linux Kernel mailing list that he is going to ensure that the rc4 is smaller by being “grumpy if people try to push as much to rc4 as happened to rc3.”
Linux 3.10-rc3 has been stuffed with mostly small changes and most of them linked to drivers related to drm, network, staging, usb, video and the likes.