Hi.
See this link for additional info on maintaining your vCenter DB: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1007453.
Personally, I'd ask your SQL admin to set up a maintenance plan to shrink and truncate your database on a regular basis. You'd be surprised what this can do for your performance. Also, check that your backup jobs are properly committing logs after a successful backup.
mike