The System Health - Excessive Data Volume metric is showing non-zero value. This means that the database loading capacity has been exceeded and it cannot keep up. So the system starts to drop data and the first data it drops are User Session Hits.
What happens is that it does not write the User Session hit record out to the database when the system cannot keep up. This does not affect the metrics for Pages, Applications, Services, etc. The System Health metric "Excessive Data Volume" indicates that this is happening. It is one of the metrics we expose to inform that monitoring has exceeded the capacity of the hardware.
This indicator does not really have anything to do with the size of the database. It has to do with the processing capacity of the system to insert new records into the database. So it could happen on an empty database or a full database.
To fix the issue, here are some options: