For some reason, the gtk2 font is different than it was some time ago. I haven't touched it, I can't remember what were the circumstances, but it has changed.
Here's a screenshot of gedit (gtk3) and firefox (gtk2) side to side: http://i.imgur.com/RCzkXvV.png
Thanks for your kind help. Note: I also haven't found anything font-related in the settings, apart from the Large Text setting in Universal Access. Is there a way to change the size of the font? Everything is too big by default.