I can't point you at anything that confirms what you are witnessing, but I can understand it (sort of).

When a transaction is written to the transaction log, it is either marked for replication or not. Parts of the log cannot be marked for re-use until all transactions marked for replication have been processed. Adding the recovery model into the mix...

In FULL recovery, although your large transaction is not marked for recovery, that part of the transaction log cannot be marked for re-use until it is backed up, and so the Log Reader has to grind its way through that part of the log.

In SIMPLE recovery, SQL Server can mark the part of the log with your large transaction for re-use, and the Log Reader can skip it entirely.

As I said, I can't point you at anything to confirm this, so this is just an educated guess as to how SQL Server goes about its businees.