A vmdk actually consists of two files. A descriptor file and a data file. The descriptor file is simply a text file that includes a brief description of the disk and a pointer to the data file. The descriptor file on the disk would be something like server.vmdk and the data file would be server-flat.vmdk. When you view the files with the datastore browser the vSphere Client displays the server.vmdk but uses the on disk size of the server-flat.vmdk. When you look at the storage from the busybox console you will see allocated space (potential size).