Hi David,
good to know that sorting solved your problem.
I understand perfectly that given the urgency of your situation, having the
solution ready takes priority over continuing with the investigations.

I would recommend anyway to open a Jira issue in Apache Solr with all the
information gathered so far.
Your situation caught our attention and definitely changing the order of the
documents in input shouldn't affect the index size ( by such a greater
factor).
The fact that the optimize didn't change anything is even more suspicious.
It may be an indicator that in some edge cases ordering of input documents
is affecting one of the index data structure.
As a last thing when you have time I would suggest to :

1) index the ordering which gives you a small index - Optimize - Take note
of the size by index file extension

2) index the ordering which gives you a big index - Optimize - Take note of
the size by index file extension

And attach that to the Jira issue.
Whenever someone picks it up, that would definitely help.

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