After one of the recent Git updates, our HTML editor went wide (see pic). Our theme is based on custom corners and this has only happened recently... weird.
It's a typical css problem - something in your theme or for example in some block css might override normal htmlarea css (margin/padding for .button, .img, .toolbar etc )
Try Firebug or Web Developer plugin (in Firefox) to check what's after normal htmlarea.css tags for that editor toolbar