Cloud cost
Incremental snapshot
A backup that stores only the blocks changed since the previous snapshot in its chain, not a full independent copy. That's why deleting one frees far less space than its listed size suggests.
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Definition
Block-storage snapshots (EBS's, and most cloud providers' equivalent) are incremental after the first one: each new snapshot references the unchanged blocks in the snapshots before it instead of duplicating them. That's what makes frequent snapshotting cheap in principle, but it also means the console's per-snapshot size figure (the full logical size of the volume at that point in time) wildly overstates what deleting any single snapshot in the chain would free.
Automated backup schedules with no retention policy are the most common way this becomes real money: nothing about an incremental snapshot's low incremental cost stops it from accumulating indefinitely if nothing ever expires it.
Where it shows up
An incremental snapshot's size appears in the console as the full logical size of the volume at that point in time, not the incremental bytes it actually stores. EBS lists each snapshot in a chain this way, which is why the per-snapshot number on screen doesn't match what deleting it would free.
What makes it expensive
An automated backup schedule with no retention policy is the usual culprit here. Each new snapshot costs so little on its own that the total accumulating underneath goes unnoticed for a long time. If the snapshot count climbs month over month and no expiration rule appears anywhere in the schedule, the chain has no end condition and will not grow one by itself.
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