Each time you make a snapshot, vCenter Server creates a new file, usually with delta
in its name. It starts writing all the changes into this file, leaving the original
.vmdk file (which represents a virtual machine's hard drive) untouched. Changes are written on the blocks' level, that is, even if you are moving a file from one folder to another (inside the guest OS), it's already considered as a change to VM and this change is added to the delta file.
This means that vCenter Server is just adding changes and increasing delta files. There is no limit for it to grow (it grows while there is free space on the storage) and a snapshot can become several times bigger than a VM's virtual disk itself.
If one more snapshot is made, the server ...