The activity log is a possible reason for this.

Because the activity log is stored in the database, the size of the activity
log must be factored into the amount of space allocated for the database.
The activity log size is dependent on the length of time the activity log
information is retained. When the length of time is increased, the amount of
accumulated data also increases and additional database storage is required

You will have to us give more information about your policies and the last
couple of days of events so we are able to asses the possible causes.

Leo Humar
LCS Pty Ltd
lhumar AT bigpond.net DOT au

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