Hi Paul,
The answer is Record History Data, depends on each record that this feature is enabled, also on the frequency that the record is updated, the count of records, and the time it was enabled. Consider a copy of the record multiplied by all of the previous factors and you will have a clue.
The solution is to analyze with one is the primary cause, disabled it or check when this problem started and If it was some hal code that is producing this problem.
Regards