ISPManager, archived copies, why are the sizes different?

Question

ISPmanager

This is how the picture looks like on almost all users in ISPmanager.
Namely: two archives have a time difference of 5 days. The size difference is always the same.
That is, the first is always greater than the second.

Why is that? Where to look to see the logic of the formation of archives?

Answer 1

this is called a differential backup
www.handybackup.ru/differential-backup.shtml
first a full copy is made, and then only the changed files.