my point was that if I had already downloaded the three sets of images (approx. 4gb), the total number of files should not increase at all
The number of files in the folder "Items", yes. But not the number of files in the folders "Persons" and "Medias". You need to clarify which folder you were monitoring. If it's the folder "Images", that folder includes the folders "Persons" and "Medias".
unless I land on one of the movies recently added to the database by you.
Yes, the set of movie posters is about one year old.