As capezotte has already suggested, you may need to clean pacman's package cache:
sudo pacman -Scc
Disk usage analyzers like ncdu (cli), filelight and baobab (gui) are very useful in your case:
pacman -Ss ncdu
galaxy/ncdu 2.2.1-1 [installé]
Disk usage analyzer with an ncurses interface
community/ncdu 2.2.1-1 [installé]
Disk usage analyzer with an ncurses interface
pacman -Ss baobab
world/baobab 43.0-1 (gnome)
A graphical directory tree analyzer
extra/baobab 43.0-1 (gnome)
A graphical directory tree analyzer
pacman -Ss filelight
world/filelight 22.08.3-1 (kde-applications kde-utilities) [installé]
View disk usage information
extra/filelight 22.08.3-1 (kde-applications kde-utilities) [installé]
View disk usage information