Just like on Debian, Arch will not clear its package cache:
$ du -hs /var/cache/pacman/pkg/
103G /var/cache/pacman/pkg/
Just like you can clear it with apt-get clean on Debian, you can do:
# pacman -Scc
However, if you only pass it -Sc, it'll only remove the package
versions you don't have installed:
[root@quanah ~]# pacman -Sc
Packages to keep:
All locally installed packages
Cache directory: /var/cache/pacman/pkg/
:: Do you want to remove all other packages from cache? [Y/n]
Adding the second c makes it remove those archives too.
Happy cleaning!
