The SegmentWriter currently buffers the list of child nodes changed on a nodestate update  (new node or updated node). This can be problematic in a scenario where there are a large number of children added to a node (ie. unique index size seen to spike above 10MM in one case).
To have a reference for the impact of this, at the SegmentWriter level, for a list of map entries of almost 3MM items, I saw it take up around 245MB heap.
This issue serves to track a possible improvement here in how we handle this update scenario.