The previous arrow is for some reason undersized on OSX Chrome 25.0.1364.160 and Safari 6.0.2. This does not seem to be the case when the fullcalendar library is used independently of the fullcalendar Drupal module.
UI still works--just looks a little funny:
Closed: outdated
User interface
Version 6.x of this module is no longer actively supported. Closing out tickets against that version. If this is still an issue in an actively supported version of this module, feel free to re-open and provide steps to reproduce the issue.