SyncBack always working

Hi, i’m looking for solve an inconvenience using my SyncBack (ver. 5) software backup.
I configurated the app to save and synchronize my data from Source, a folder (Documents) that includes many subfolders that contain some midi files too, to Destination, an exact integral copy of first folder (Documents).
The used parameters are f.e. to overwrite from Source to Destination and enable an intelligent synchronization also when i modify something on Source the app will modify on Destination.
The problem is that when the synchronization is enabled the app tries to copy not only that one to copy by modification but detects changes in a subfolder containing .mid and .kar files and works continuing in a loop.
One time is appeared me a window by SyncBack where i noticed that same .mid files on Source were of different sizes than Destination (and even oddly some sizes on Destination were bigger than the same file on Source!).
It is as if the app detects sizes files as not stable…i suppose.