After running a full system upgrade on Arch Linux, all of my desktop environments save MATE were using a massive font size. This made all the windows balloon up as if I were running with 640×800. Part of the upgrade must have damaged/corrupted/replaced a global font config file (if any such thing exists).
The solution to get back to the normal font DPI (96 for me). Is to load System Settings > Font (at the top) and then “Force Fonts DPI: