The page is as wide as its largest element which in the case of that page is the large picture in one of the posts.
Sometimes it can be caused by a long URL and on others it can be done deliberately by typing lots of letters without a space.
i.e. zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz going on for miles.

this can be fixed by the original poster(or section mod) changing the size of the picture with {img width=x height=y}img url here {/img} replace x and y with appropriate numbers(400/200?) and {} brackets with [] ones