Transaction log backup file larger than expected?

I've recently taken over a SQL Server database that didn't have any transaction log backup routine running for at least 2 years. The first log backup was very large (3.8 gb on a 1.14 gb database) as you would predict, however subsequent log backups are still larger than I would have expected (somewhere in the region of 3 mb).

I also restored the database to a QA and ran several sequential log backups without there being any activity on the database and the log file backups were still ~3 mb.

What would account for the size and consistency of the transaction log backups with nominal activity on the database itself?