Index disk size is high during initial index build

Build 5.1.2-6022
Coming from https://issues.couchbase.com/browse/MB-30855?focusedCommentId=289810&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-289810

Observed that while doing initial index build, index disk size goes high and then settles down after some time.
For a test of 500M docs, single index build-
5.1.1-5723 takes maximum of 40GB space, whereas 5.1.2-6022(and vulcan, alice) takes ~100GB space at peak.
Also observed that higher percentage of fragmentation while initial index build.

Here is cbmonitor graph- http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=nyx_511-5723_build_secondaryindex_5233&snapshot=nyx_512-6022_build_secondaryindex_8740#ac9466ec456c2a28c48d40e1c4725c3e