Incorrect database size (and growing)

On the privacy settings page (index.php?module=PrivacyManager&action=privacy) :
Current database size:104.8 M
Estimated database size after purge: 104.4 M
(old data deltion) Daily
However the actual disk size of the database reported in the SQL manager is currently 250 MB
I tried the purgeDataNowLink, but the actual database size on disk has not changed.

This happens on 2.16.1, traffic is low (50 visits daily), database is small and I would like it to stay like this without any mysterious leftovers on disk.

Wow, 2 weeks and no support. Is there any way to manually delete traffic data?