The preview is really for you to visually double check your post for grammar/typos, format issues, BB codes and other malformed items in your post before submitting. The preview width is never guaranteed to be the same as what is actually posted because depending on where your post ends up within the thread, the width and height will naturally have to adjust to the nesting/depth in the thread. I hope this makes sense.
If I answer a post I'm filling in a box that's lets say is 6 inches in width.
If I preview the post it displays in a box that is 5 inches in width.
This might give the post a different look.
Why is the screen/box size different?