I assume you re-index in full right ?
My shot in the dark is that this increment is temporary.
You re-index, so effectively delete and add all documents ( this means that
even if the new field is just stored, you re-build the entire index for all
the fields).
Create new segments and the old docs are marked as deleted.
Until the background merge happens, the index could reach those sizes.

the weird thing is why the merge didn't kick in...
Have you configured any special approach in segments merging ?

What happens if you explicitly optimize ?

Let us know ...
Alessandro Benedetti
Search Consultant, R&D Software Engineer, Director
Sease Ltd. - www.sease.io
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