Have you added any images that have pushed the width of the topic box? I have tended to find that has caused this problem!
Also view source and look for a string of non-breaking spaces & n b s p ; (without the spaces) or words with non-breaking spaces between them.
Another check would be the side blocks, if you remove all of the sideblock on either the left or the right side of the screen the topic section uses the space the side blocks would have.
Cheers
Teresa
Cheers
Teresa