PGSQL file sizes

We've had AD Audit running for some time now and it seems to be using a lot of disk space.

Was hoping someone could point me in the right direction?

Archiving is turned on (190 days) and every option is checked.
I can see a zip file named "AUDFileAuditInfo_1455258195464.zip" dated 02:00 this morning (17Mb) so this would appear to be working.
Folder C:\Program Files (x86)\ManageEngine\ADAudit Plus\pgsql\data\base\ is 40GB in size with files from today all the way back to Sept 2014
The majority of this space seems to be taken up by 57 files over 400Mb each, the oldest being from 20th February 2016 (184 days ago)

Any ideas why only 57 of these files from a total of 4,341 are so large? (400 to 600 Mb)