HTreeMap uses more direct memory than specified in HashMapMaker.expireStoreSize()

[igalkin]

Hi, I am using MapDB v3.
I am setting store size limits using HashMapMaker.expireStoreSize() combined with expireAfterCreate() and expireAfterUpdate()
What I see is that I am able to store a single data item above this limit before older items get evicted - eviction happens on next update (using a foreground evictor).
How do I prevent data stored above the memory limit, besides just specifying limit lower by a safety margin and ensuring single item size doesn't exceed a safety margin ?