40GB of thumbnails in a 16GB device
The problem was ultimately fixed by ignoring the whole /sdcard/DCIM/.thumbnails directory in the synchronization.

Deleting the sparse thumbnails was useless as they will grow again but are basically waste. Another interesting point should be telling FolderSync not to sync hidden directories (all caches are hidden) but that will not save directories such as Boldbeast Recorder's .BBRec folder or, in general, any data folder incorrectly marked hidden by the creator app.