After the last archlinux update, my cursor was about 3 to 4 times bigger than normal. First it only occurred in skype, but after a reboot it affected the cursor everywhere.
After searching a bit I found this post in the Arch Linux forum. The most practical answer is to comment out the Inherits=Adwaita, which is set through new dependencies:
Then restart X and your cursor should be normal again. The problem only occurs, when no other theme is set, so it’s likely that only users with basic/uncommon display environments are affected (like in my case xmonad).