did you notice that today, since moodle.org is running v3.4, the TinyMCE editor (https://docs.moodle.org/34/en/TinyMCE_editor) has a smaller font while editing?
I checked on the QA testing site and found the same smaller font while editing using TinyMCE, so realised the problem wasn't moodle.org specific. I then checked on the Sandbox demo site (running 3.3) and found the same again, so realised the problem wasn't 3.4 specific! I didn't find a tracker issue for it though.
Thanks Samuel for confirming. Would you or Matteo like to create a tracker issue for it? Otherwise I can.