Github user afs commented on the issue:
https://github.com/apache/jena/pull/209
Space saving: intentional!
[JENA-1147](https://issues.apache.org/jira/browse/JENA-1147) and
[background
email](http://mail-archives.apache.org/mod_mbox/jena-dev/201603.mbox/%3C56DB0C83.2040609%40apache.org%3E).
While not free in terms of pure paring speeds, up to a 50% space saving is
very useful.
And at Java9, there will be saving due to [JEP 254: Compact
Strings](http://openjdk.java.net/jeps/254).
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