Online clustered index rebuild using more log space than expected
After contacting Paul Randal of SQLSkills he informed me that the reason for the high rates of transaction log usage is that in an ONLINE index re/build each row has it's own log record with all the overhead of a log record. An OFFLINE rebuild logs full 8kb page images, not row by row.

Related link provided by Paweł Tajs:

SQL Q&amp;A: The Lore of Logs (also by Paul Randal)