Diane,
Really not sure. We haven't played with the Fan box in that wide of a view before, but it may simply be Facebook shrinking it to the more 'normal' size of the Fan box that's used (for sidebars and such). You can try using the FB Like-Box configuration tool, on the page below, to see if you get the same results. From what it looks like, Facebook always uses a max-width of about 80-100px, so has some non-standard proportions, it can get squished how you're seeing:
developers.facebook.com/docs/reference/plugins/like-box/
Let us know how it goes, but unfortunately not anything we can do to control how the actual posts in that widget are displayed.
Thanks,
Alex