Transaction log backup file larger than expected?
Having spent some time looking at this in more detail it would appear that the unusually large transaction log backups are due to excessive internal fragmentation of the transaction log itself. This is due to unrestricted autogrowth of the transaction log over a long period of time.

The following excerpt from a SQLSkills.com article more precisely defines the problem


  Often, when transaction logs are not pre-allocated and/or when there's
  been a lot of autogrowths, the transaction log can become internally
  fragmented. Internally your transaction logs are broken down into
  smaller more granular chunks called VLFs (Virtual Log Files). The size
  and number of VLFs you'll have depends largely on the size that the
  chunk is when it's added to you transaction log. If you add a new
  chunk to the transaction log which is 20MB (through autogrowth or
  through manual growth) then the number of VLFs that are added is 4. If
  you add a chunk which is greater than 64MB but less than or equal to
  1GB, you'll add 8 VLFs. If you add more than 1GB then you'll add
  16VLFs. In general, most transaction logs will only have 20 or 30 VLFs
  – even 50 could be reasonable depending on the total size of the
  transaction log. However, in many cases what happens is that excessive
  autogrowths can cause an excessive number of VLFs to be added –
  sometimes resulting in hundreds of VLFs. Having an excessive number of
  VLFs can negatively impact all transaction log related activities and
  you may even see degradation in performance when transaction log
  backups occur. To see how many VLFs you have solely look at the number
  of rows returned by DBCC LOGINFO. The number of rows returned equals
  the number of VLFs your transaction log file has. If you have more
  than 50, I would recommend fixing it and adjusting your autogrowth so
  that it doesn't occur as fequently. To get rid of all of the
  execessive VLFs, follow these easy steps to shrink off the fragmented
  chunk and add a new, clean chunk to your transaction log.


On this particular database it turns out we had 3011 VLFs, some degree over the recommended limit of 50 :)

We are now working through the steps in the above guide to reduce the number of internal VLFs. The QA database has been shrunk and the start size set to 2.5 gb, we now have 23 internal VLFs. So the resulting TLOG backup is in the region of 128 kb.