VMware has released a useful KB article to help you work out where your vCenter database growth may be coming from. http://kb.vmware.com/selfservice/search.do?cmd=displayKC&docType=kc&externalId=1028356

As the vCenter database is the only place for storing all config information, performance data, tasks, events etc. it can grow very quickly especially if you are doing large scale deployments.

The article may point you in the right direction and highlight if you are gathering too much information and/or not purging old data.

You can have a look at your vCenter Server Settings and look at the Statistics and Database Retention Policy settings to see if perhaps you are gethering too much information.

As vCenter becomes critical having a single database holding everything makes your infrastructure management tool too cumbersome.

I would really like VMware to split out the tasks/events/performance data from the critical core configuration/operating data and store it in a separate database so when you have to fix your core installation you are not faced with a massive database of non critical information to work with.