How backup tools reduce size and time.
FULL
A complete copy every time. Simple, large, and slow, but a single archive contains everything and restoring is straightforward.
INCREMENTAL
Only what has changed since the last backup of any kind. Smallest and fastest, and restoring requires the last full backup plus every increment since, in order.
If one increment in the chain is corrupt, everything after it is unusable.
DIFFERENTIAL
Everything changed since the last full backup. Larger than incremental, and restoring needs only the full backup plus the most recent differential.
WHAT THIS MEANS PRACTICALLY
Most website backup tools take full backups, because websites are small enough for it to be practical.
Server-level tools frequently use incremental approaches, which is why hosting backups appear to consume less space than you would expect.
WHAT TO PREFER
For your own backups, full copies. They are simpler, restore more reliably, and the storage cost at website scale is modest.
Complexity in a backup system is a risk, because it is one more thing that can fail at the moment you need it.
