Starting the task from the server console has the same result. Starting the task with the -c option gives the expected result, recovering something like > 20%.
Subject: default compact is not recovering whitespace
If you run ‘compact’ the server in fact does a ‘compact -b’. This consolidates whitespace in the database, so that it can be reused efficiently, limiting further unneccesary growth of the database. It does however not reduce the filesize, as you’ve noticed. For filesize reduction, use compact -B or as you say, compact -c.