Are you using thin or thick provisioned VMDKs?  If you created your VMs with thick VMDKs, you'll have VMDKs that are big in VMware, but will be seen as empty by the Compellent. Compellent doesn't write zeros and will show that space as free. 
This datastore shows 231GB free in vCenter and 760GB free in Compellent (see attached)  How can it show more free in Compellent than in vCenter? Since Compellent does everything by Thin Provisioning?  I get the feeling I am missing something hahaha I mentioned it to Compellent support too, the guy had a look and couldn't quite see why and he is going to chat to some other people and get back to me. I am sure it must be something simple I am missing? [IMAGE][IMAGE]
Free space discrepancy between esxi/vCenter and Storage Manager