I've seen this issue here.
Media Drive: 4TB WD Red (a 5400 rpm drive)
Files in Cache (MB and Cache are on a SSD): 11,067.
-- 11,024 of them were created the day I upgraded to 1.4.0. The 43 covers added to cache since seem to be in line with what I've added and adjusted in that time. Maybe some of my miscellaneous stray tracks don't have art, don't know if it's ~900 of them though. It certainly could be.
I run the same copy of MB from a networked directory on three different machines if that matters.
I've gone down to 1.1.0. I admire that you're adding to the plugin, but I don't need all that functionality. I basically want to just be able to look at the art for the current track, play/pause/skip/etc, rate tracks, and maybe rearrange the now playing queue or look at lyrics sometimes. That's all in 1.1.0. The one feature I miss is "stop after current," but that's about it.
A smile is happiness you'll find right under your nose.