Public Folder Source Agent on Target Side (Exchange 2013) extracts the same data over and over, though no changes are made in the target Public Folder tree. If you open the Associated Contents of one of the target Publci Folders with MFCMAPI, you'll see a lot of hidden messages having subject which starts with AelitaEMW.
Exchange 2013 Public Folder store PR_EntryID property value exceeds 245 characters. (Can be viewed with MFCMAPI).
1) Install Migration Manager latest update 20150320EX which can be downloaded from: https://support.quest.com/kb/77417
2) Run the PF Stream Cleaner attached, to cleanup the hidden messages from the target Public Folders. Please use the document supplied with the tool as a guide for installation and usage.