Welcome to the Community,
did you select the same "Deployment Size" as you did for the original VM, and are you using the same vCSA version/build as the one used for the backup?
Thanks for your reply. Yes, I do use the same version and size. I have double checked this. I notice that even though I select Small as the Deployment Size, the only option for Storage Size is X-Large. I guess that may be because it needs that much space to store the content of the backup.