Comment 2 for bug 1000726

Note that the big win here is if you have documents that change a lot, but not a lot of documents. As a limit case, imagine 1 document, changed 10000 times. The transaction log would be 10k long, but after purging, you only have 1 doc_id you need to store.
As the other limit case, 10k documents, only created (never modified). Purging the log will just move where the doc_ids are stored, not actually saving any space (but maybe saving some index meta-info space, etc.)