I migrated two site collections sequentially using a 3rd party tool from SP 2010 to SP 2013. The fist site collection was almost 200 plus gb on source and became 80 GB on destination (thanks to shredded storage and clean up). The second site collection was 83 Gb on source but on destination it took 125 GB. What could be the reason for this size increase? I then move the 2nd site collection to a different content DB (as the combined site collections had crossed 200 GB mark on destination). Post moving the new content Db just having 2nd site collection is of 145 GB. What is the reason behind this growth? There is no data being added from users end.
The database structure changes between versions of SharePoint, thus, the changes in schemas mean migrating data to a new format within SQL. There will be a lot of blank space within those DBs that can be cleaned up once migrated.
For reference: Technet