I've discovered a bug where if a user changes text on their page from small to larger, then hits preview, then save, the change is stripped out. The span tag that was added during the format change is there, but the inline CSS style is removed. The change was visible during preview! It was only stripped/removed after clicking save. This bug only seems to apply to regular page authors, not the administrator (uid1).
I verified that the input format is set to filtered and that the filtered input format does NOT have the option checked to remove CSS styles.
I'm using the latest version of the module:
CKEditor - WYSIWYG HTML editor 6.x-1.10+10-dev (2012-Mar-23)