Hello!
I re-set up my Pi maybe 3 or 4 months ago, from scratch.
My library, has always been the same (approximately) - over 2000 films and around 70+ TV shows.
I used to regularly, clean my library.
None the less I recall my thumbnails, being so big about 18 months ago, that I thought it was in excess of 6GB maybe 8 or so? It's hard to recall.
Have the TVDB / Movie Database site moved to higher compressed source thumbnails or is libreelec somehow compressing?
Maybe it's just luck? I don't know, I'm happy about it but surprised.