Used Storage decreases on SP 2013

Hi there,

we migrated a SharePoint 2010 SiteCollection to SharePoint 2013 Hive 14 (in first step).
The site-Collection has a storage of about 200 GB on SharePoint 2010.
Most storage is used for document libraries.
After the migration to SP 2013 (2 weeks ago), the storage has deacrease to 160 GB.

Is this or normal behavior or do we have any trouble with the site-collection ?

Thanks
Stephan