You may have noticed recently that some Facebook status updates are in a larger font size than you may be expecting. Facebook has - without really announcing it - added this as a new feature. If your update is less than 35 characters in length, it is presented in large font, like this:
It's a bit on the irritating side, especially if you're not aware that it's going to happen. There's a few things that you can do to ensure that this doesn't happen. First of all, add in more characters to get you over the 35 character limit. Second, add in an image. Third, add in a white space image, for those times when you don't have an appropriate image to add. One of those methods should do the trick for you.