pacman keeps a cache of package versions it installs in /var/cache/pacman/pkg/.
As you install and upgrade more packages, the more it grows.
So, it’s a good ideia to clean it from time to time.
If you want to keep the last 2 versions of each package as an example, just execute the following.
sudo paccache -rk 2
==> finished: 2670 packages removed (disk space saved: 8.49 GiB)