It looks like you have two ubb forums running.
But the two together would be below the 350MB.
I would think.
Since none of us know what else you have saved to your site?
Gizmo's statement about the logs is correct about the minor changes in file storage but it could be a culprit meaning that old logs need to be pruned.
Or you could also have some native email accounts that need to be cleaned out.
What you really need to do is use the control panel in conjunction with FTP and find what you have on the site.
And do some housekeeping.
