Q: Why would deleting an incremental snapshot increase the space usage & cost? Google Cloud Platform
From the Google Cloud docs: https://cloud.google.com/compute/disks-image-pricing#persistent_disk_snapshots

When you delete a complete or incremental snapshot, some of its data may move to the next incremental snapshot in the snapshot chain. This additional data increases the storage cost because you are using more space in the storage system.

In the Google Cloud docs they provide an example of how incremental snapshots are stored: https://cloud.google.com/compute/docs/disks/create-snapshots

Snapshot 3 contains any new or changed data since snapshot 2 but won't contain any unchanged data from snapshot 1 or 2. Instead, snapshot 3 contains references to blocks in snapshot 1 and snapshot 2 for any unchanged data.


Amazon Web Services
However, to better help illustrate the AWS docs have an example scenarios of deleting an incremental snapshot: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-deleting-snapshot.html

Deleting a snapshot might not reduce your organization's data storage costs. Other snapshots might reference that snapshot's data, and referenced data is always preserved. If you delete a snapshot containing data being used by a later snapshot, costs associated with the referenced data are allocated to the later snapshot.


AWS states that deleting might not reduce storage usage & costs. Although it doesn't say storage & cost would increase.
Why and how would deleting a snapshot in GCE increase storage space & cost?

A: It results from incremental nature of snapshots.  Let's consider we have three snapshots:


*

*snapshot1: has data blocks A, B, C

*snapshot2: block D was added, block C removed. Due to incremental nature this snapshot does not store A and B.

*snapshot3: block E added, and info about this block only is stored here.


Whichever snapshot you delete, the remaining ones will have all necessary info to restore full data on that moment. For example, if you delete snapshot2, info about added block D and removed block C and will be assigned to snapshot3. 
Thus, you'll free no storage.
