Most of the size will be due to the performance data that you capture. The frequency of the data capture is configured in the VC client - go to Administration\Virtualcenter Management Server Configuration. Then select the Statistics page. If you change Collection Intervals, you can reduce the size of storage space you need. For example, you made not need the past years worth of data (up to the last month may be fine) and you can then delete that collection interval. You should also note that when you create a collection interval, you can set the detail level which will impact the amount of data storage space required.
If you need to you can manually delete data from the database. There's a good write up on that and managing collection intervals here: http://www.vmware-land.com/Vmware_Tips.html#VC4
