Hosting the kernel git repositories has many more implications than simply managing a raw number of contributors. The kernel repostiory and source is much, much larger than Ruby on Rails.
linux-3.1-rc3$ ls -R -l |wc -l
46855
linux-3.1-rc3$ du -h -s
500M
rails$ ls -R -l |wc -l
4141
rails$ du -h -s
59M .