WT-6082 worked towards storing the validity window on the data files only when really needed. The driving workload behind that improvement was WiredTiger's simple operations using wtperf. wtperf doesn't use timestamps and only relies on the txn-ids. We noticed that we were storing txn-ids on the data files all the time, causing a performance hit and much larger data file sizes. Having fixed WT-6082, we got the expected simple operation's performance back and reduced the data file sizes.
WT-6082, in 4.4-rc5 Mark Callaghan reported _id index sizes that were 3 to 4 times larger than 4.2.6. After WT-6082, in 4.4-rc6 the reported _id index size is 2 to 3 times larger. We need more work in investigating why we still have a larger index size than 4.2.6 and if we can do something to resolve that. I have filed this ticket to look into this issue.