Automatic backup files size doubling with no reason

Moodle 3.5.3  & PHP 7.2 & Maria DB

I have found that the automatic backup files size has been doubling  at certain dates with no KNOWN reason ( see attachs).

I have not made any changes during last days, only updating Moodle version

Courses have only quizzes, so students are not uploading content.

Any hint would be  greatly appreciated