check the following VMware KB Articles to get a better understanding about VMware Snapshots:
How much space you need to successfully commit depends on the actual size of your delta file ….delta.vmdk.
If you can try to commit the snapshot when the virtual machine is off. With this you avoid the need to create another helper snapshot for the VM to wite to when merging the delta disk with the base disk.
Also the space requirements needed to commit a snapshot have changed in the last 3 versions of ESX Server, so it also depends what version of ESX Server you are running.
This Article disribes this: