Moving the thumbnails to SD doesn't actually address the OP's problem...jut shifts it to SD
I would assume deleting the thumbnails folder together with Textures*.db should take care of the folder size. Your XBMC will run slower after the reboot as the images will have to be cached again as needed.
I have seen Python scripts to regenerate the thumbnails faster based in installed add-ons, but I don't know how to install Python on AFTV.
4.4.2 Stock, not by choice
Stock, rooted >>>> replaced by AFTV
, you guessed: stock
4.2.2 Stock, rooted...can you say I am...STOCKED?